Molten NaNO3 Research Articles

The thermal conductivity of molten NaNO3 and KNO3

The thermal conductivity data for molten NaNO3 and KNO3 have been examined in order to propose recommended data sets for these two popular heat carriers and to establish the reference values above the temperature range covered by toluene and water. It is known that the measurement of the thermal conductivity of molten salts is very difficult, owing mainly to their corrosiveness and high melting temperatures, which introduce complications in apparatus design and significant systematic errors due to radiation and convection. However, some recent measurements seem to manifest more trustworthy values than obtained before. All available data have been collected and critically evaluated. The temperature range covered is 584 to 662 K for molten NaNO3 and 662 to 712 K for molten KNO3, with the confidence limits better than ± 5%.

Thermal diffusivity measurements of molten salts using a three-layered cell by the laser flash method

A simple cell and easy data processing are described for measuring the thermal diffusivity of a liquid sample at high temperatures using the laser flash method. A cell consists of a liquid sample sandwiched by two metallic plates. The front surface of one metallic plate is exposed to a single pulse of beam laser and the resulting temperature rise of the back surface of the other metallic plate is measured. The logarithmic analysis proposed by James using the initial time region of the temperature response curve of a two layered cell system has been extended to apply to the present three layered cell system in order to estimate the thermal diffusivity value of a liquid sample. Measurements of distilled water and methanol were made first and the results were found to be in good agreement with the reference data. Then, the thermal diffusivities of molten NaNO3 at 593–660 K and of molten KNO3 at 621–694 K were determined and the results also appear to agree reasonably well with those reported in the literature.

Study of iodide-iodate isotopic exchange reaction in a few eutectic melts by radiotracer technique

The isotopic exchange of iodine between iodide and iodate ions was investigated in molten NaNO 3-KNO 3 eutectic and iodide-iodate melts using 131I as radiotracer. This exchange reaction obeys the rate law R = k[I −][10 3 −] 0.8 at 340°C in eutectic molten nitrate. The activation energy is found to be 145±5 kJ mol −1. The activation parameters, i.e. free energy of activation, enthalpy of activation and entropy of activation, were also calculated. The exchange in homogeneous melts of NaINaIO 3 and KIKIO 3 systems is found to be very fast, probably instantaneous at 5°C above the melting temperature. No noticeable exchange is observed even after heating the samples for 30 min at a temperature 20°C below the corresponding melting point. These results support an association-dissociation type reaction mechanism similar to that proposed earlier for this exchange reaction in neutral aqueous medium.

Vibrational line broadening in molten nitrate mixtures: A static model for the coordination number and concentration fluctuations

The isotropic Raman spectra of the ν1(A′1) mode of NO−3 are measured in molten NaNO3–TlNO3 mixtures at various concentrations (the mole fraction of NaNO3, 1≥CA≥0) at 600 K. Vibrational correlation functions in mixtures at CA≂0.8–0.5 show a considerable contribution of inhomogeneous vibrational dephasing. A model is presented which accounts simultaneously for the coordination number fluctuation and the concentration fluctuation at the reference anion. Concentration dependence of the isotropic Raman band shape (peak frequency, bandwidth, and asymmetry) is fully explained by the static model for the molten NaNO3–TlNO3 and LiNO3–RbNO3 mixtures. The bandwidths at mole fractions of about 0.5, slightly smaller than those calculated with the static model, suggest a motional narrowing due to diffusion dynamics. The model also makes predictions on the concentration dependence of the spectral line profile in terms of the interaction with the nearest neighbors and diffusion dynamics in liquid mixtures.

Thermal conductivity measurement of molten salts by the transient hot-wire method. (2nd report. Measurements of molten NaNO3 and KNO3 using ceramic-coated probes).

The present paper reports absolute measurements of the thermal conductivity of molten salts by the transient hot-wire method using newly developed ceramic-coated probes. Al2O3 has been employed as the insulation material and the following techniques have been utilized to produce insulation layers : ion plating for a thin platinum hot-wire and plasma-spray for thick titanium struts. The measurements have been performed on pure NaNo3 in the temperature range 311°C to 388°C and KNO3 349°C to 439°C. The experimental data have an estimated accuracy of ±3%. In comparison with the present results, most of he previous experimental data obtained by steady-state methods show higher values which may be due to the presence of convection and radiation.

Spin Echo Self-Diffusion Measurements in Molten Salts

An NMR probe head for diffusion measurements in molten salts at temperatures up to 673 K is described. The spin echo technique using pulsed magnetic field gradients was employed. Resulting self-diffusion coefficients of Na+ in molten NaNO3 in the temperature range 596-670 K are compa­red with literature data obtained by other methods.

Solid-solid interaction between ferric oxide and sodium nitrate and thermal stability of the sodium ferrites produced

The interaction between α-Fe2O3 and NaNO3 was studied using DTA, TG, DTG and X-ray diffraction techniques.The results obtained revealed that ferric oxide prevented the thermal decomposition of sodium nitrate, even when heated for a long period at 500 ° C, owing to the formation of some type of solid solution of Fe2O3 in the molten NaNO3. When these solids were mixed in equimolar ratios they underwent solid-solid interaction at 700 °C yielding well-crystallized NaFeO2 (tetragonal) and α-NaFeO2 (hexagonal) phases. Heating at 800 °C resulted in the thermal decomposition of most of the NaFeO2 produced and led to the formation of a small amount of β-NaFeO2 without affecting the crystallinity of the α-NaFeO2. The α- and β-NaFeO2 phases disappeared on heating at 900 °C, while well-crystallized NaFeO2 (tetragonal) and Na10Fe16O29 phases appeared. Heating at 1000 °C brought about a complete conversion of NaFeO2 into well-crystallized Na10Fe16O29, which decomposed completely at 1100°C giving α-Fe2O3 and NaFeO2 (tetragonal). However, the ferrite produced suffered some type of deformation.Similar results were obtained for Fe2O3 and NaNO3 mixed in a molar ratio of 2:1 ; however, β-NaFeO2 was not produced and Na10Fe16O29 was stable even when heated at 1100°C.

Voltammetric investigations of the oxygen electrochemical systems in molten sodium nitrate at 420°C

A reaction pathway for the electrochemical oxidation of oxide anions at a platinum electrode in molten NaNO 3 was deduced from the analysis of cyclic voltammograms. The mechanism involves one diffusion step followed by a charge transfer step leading to a primary intermediate adsorbed oxygen species [O(−I)] which, in another step disproportionates into a second intermediate adsorbed oxygen species [O ads], which further desorbs as O 2(g), and oxide ions. These latter are thus regenerated by catalytic process.

Structural determination of molten NaNO3, NaNO2and their eutectic mixture by molecular dynamics simulation and X-ray diffraction

Molecular dynamics (MD) simulation was carried out for the molten NaNO3, NaNO2 and their eutectic mixture systems to compare their simulated structures with those determined experimentally by X-ray diffraction. As for the pure NaNO3 and NaNO2, parameters for repulsion potential were adjusted so that the structure factors of their simulated systems coincide reasonably with the corresponding experimental ones. The MD simulation of the mixture was carried out with the parameters determined for the component salts. The structure factor of the simulated mixture system successfully reproduced the experimental one, which shows validity of the proposed pair potential. The assignment of most of the peaks in the experimental correlation function was performed with the aid of simulated results. The simulated data has been used to determine the diffusion coefficients of the component ions.

Cathodic reactions involving lead, cadmium and silver ions in NaNO 3-KNO 3 melts at 250°C

Cyclic voltammetric studies of Pb 2+ and Cd 2+ ions on Pt and Hg electrodes suggest that the cathodic deposition of the metal does not occur in NaNO 3-KNO 3 melts. The new cathodic wave produced by the addition of these metal ions to the melt is due to the irreversible reduction of the nitrato complex, ie (PbONO 2) + + 2 e − → PbO + No − 2. In contrast to metal deposition reactions, there is no oxidation state change for the metal ion since only the nitrate ligand is reduced. Irreversible cathodic waves were observed for Pb(NO 3) 2 additions to molten NaNO 3-KNO 3 over the concentration range of 2.0 × 10 −4 to 2.0 × 10 −1 m. Fast potential scan experiments from 1 to 1000 Vs −1 using a Pt microelectrode gave no indication of any Pb or Cd metal deposition. The exhaustive electrolysis of 0.10 m Pb(NO 3) 2 and 0.12 m Cd(NO 3) 2 in molten NaNO 3-KNO 3 followed by atomic absorbance analyses of the mercury cathodes showed that the amount of metal deposition is insignificant. An investigation of Cd 2+ ions in NaNO 2-KNO 3-NaNO 3 suggests a similar reduction of nitrato or nitrito complexes rather than metal deposition. Silver ions are unique in that the reversible metal deposition reaction readily occurs in nitrate melts. This is due to both the small electric field of this univalent cation and its favorable thermodynamics that places the reversible potential for the Ag +/Ag couple significantly more positive than the potentials required for the reduction of nitrate melts.

Ultrasonic and Hypersonic Spectroscopy of Alkali Nitrate Single and Binary Melts

Sound propagation in the molten alkali nitrate single salts, LiNO3, NaNO3, RbNO3 and CsNO3 and the binary melts, NaNO3–LiNO3, −RbNO3 and −CsNO3, have been studied over the frequency range of 5–25 Mhz and 3–8 GHz by means of pulse and Brillouin scattering methods, respectively. A dispersion of sound propagation is observed at the frequency between ultra- and hyper-sound region for KNO3, RbNO3 and CsNO3. The thermodynamic properties derived from the sound velocity are approximately additive for the binary melts investigated. The bulk viscosity, ηB was derived from the absorption coefficient determined. The value of ηB/dT is negative and temperature dependence of ηB/ηS is insignificant for the molten alkali nitrate single salts. The bulk viscosity of the binary melt decreases rapidly with the addition of the component having smaller bulk viscosity in pure salt.

Formation of a refractive index distribution profile in glasses due to ion exchange

The paper studies the refractive indexes radial distribution for various samples and its dependence on the conditions of ion exchange in order to obtain large aperture self focusing elements with a n(r) distribution in the form of a hyperbolic secant or a parabola. The spatial distribution obtained for alumoborosilicogermanate glass immersed in molten NaNO /SUB s/ is shown. The authors show that processing of potassium glass in molten lithium nitrate can be used in making elements for integral optics, where smaller gradient layers are required.

Raman study of rotational motion and vibrational dephasing dynamics of NO3-in molten nitrates

The Raman spectra of the symmetric stretching v 1(A′1) mode of NO3 - ions have been measured in a series of molten nitrates, LiNO3, NaNO3, KNO3, RbNO3, CsNO3, AgNO3, TlNO3, at a temperature 10–20 K above each melting point. Temperature dependence of the Raman spectrum was also measured in molten NaNO3. Reorientation of the symmetry axis of an nitrate ion is represented by the inertial motion of an associated ion cluster for short times and by rotational diffusion for longer times. One observes a point of inflection between two types of rotational motion at 0·2–0·5 ps. The speeds of inertial rotation and rotational diffusion decrease in the order, CsNO3, RbNO3, KNO3, NaNO3, TlNO3, LiNO3, AgNO3. The ratio of the effective moment of inertia obtained from moment analysis to the theoretical moment of inertia for a free NO3 - ion, I/I 0, increases in the same order, from 2·0 for CsNO3 to 20 for AgNO3. The vibrational correlation functions are analysed on the basis of a model of simultaneous homogeneous and inho...

Study of the thermal decomposition of MSO 4 (M = Zn(II), Cd(II), Hg(II)) in a sodium nitrate—potassium nitrate melt

MSO 4 (M = Zn 2+, Cd 2+, Hg 2+) dissolves in the molten NaNO 3—KNO 3 eutectic and is decomposed on further heating. The kinetics of decomposition have been studied at different temperatures. The decomposition of CdSO 4 and HgSO 4 in the eutectic melt obey first-order kinetics whereas the decomposition of ZnSO 4 at 420–460°C obeys second-order kinetics. However, at 480°C the decomposition of ZnSO 4 obeys first-order kinetics. The mechanism of decomposition has been given as M 2+ +SO 2− 4 +Na + +K + +2NO − 3 ⇌ (Na,K)SO 4 + M 2+ +2NO − 3 M 2+ +NO − 3 → MO+NO + 2 NO − 3 +NO + 2 → NO 2 + 1 2 O 2 Some of the end products have been analysed by X-ray diffraction.

Fabrication of single mode glass waveguides by electrolytic release of silver ions

We report a novel electrolytic process that is used to fabricate reproducible, low loss, single mode waveguides. By precisely controlling the current pumped through high purity Ag and Pt electrodes in molten NaNO3, as well as its duration, a high degree of accuracy over the release and hence the control of Ag+ concentration is achieved. The inherent capability of the process to generate very low level of silver ion concentration with high precision implies much higher time of diffusion, thus removing the time criticality factor faced by the existing processes while fabricating single mode waveguides.

MARANGONI FLOW VELOCITY IN A THIN DISK OF MOLTEN NaNO2

Abstract Temperature gradient driven Marangoni flow has been investigated experimentally in circular disks of molten NaNO3. 1.5 mm high by 7 mm diameter. Flow velocities were evaluated from motion picture film and temperatures were determined with a miniature thermocouple. The results are compared to an order of magnitude analysis and agreement is reasonably good.

The applicability of a nitrate melt as a cell electrolyte in a flue gas desulphurization cell

This paper presents equilibrium and kinetic studies of the O 2 and O 2/SO 2 electrodes in molten NaNO 3-KNO 3 eutectic at 380°C. For the O 2 electrode the overall reaction is given by the equation: 1 2 O 2 + 2 e − = O 2−. The exchange currents for this reaction are higher with platinum than with gold and are 1–2 orders of magnitude higher than those previously measured in molten sulphates. The overall reaction of the O 2/SO 2 electrode is: SO 2 + O 2 + 2 e − = SO 2− 4. Gold is a better catalyst than platinum for this reaction, quite similar to the case in molten sulphates. Molten NaNO 3-KNO 3 seems to be an appropriate electrolyte for use in a new electrochemical flue gas desulphurization process in the temperature range of 350–450°C.

Mesure de la vitesse du son sous pression dans les silicates liquides. Mise au point de la méthode sur un sel fondu

An impulsional method is described for the measurement of sound velocity in liquids as functions of pressure and temperature. From sound velocities under pressure and heat capacities and molar volumes at atmospheric pressure, an iteration calculation yields the density, the compressities and the heat capacities as function of temperature and pressure. The method is applied to molten NaNO3.

Nuclear spin relaxation in molten alkali nitrates

The nuclear magnetic relaxation of the 23Na and 87Rb spins in molten NaNO 3 and RbNO 3 was measured. The rates were found to be 19.2 and 670 sec −1 near the melting point and decrease in proportion to the reciprocal temperature. The rates were explained by means of the quadrupolar mechanism, with electric field gradients calculated from a model consistent with the observed field gradient in solid NaNO 3. Calculation of the field gradients around the triangular nitrate ion coupled with X-ray diffraction measurements led to the conclusion that the translational motion of the ions was the most effective relaxation mechanism with a relatively smaller contribution from the rotation of the anion. Under these circumstances the correlation time calculated from our data is the average time that the spatial configuration of a group of near neighbors persists in the liquid state. It was found to be about 8 × l0 −13 sec.

SEM/EDX and AA Measurements of Current‐Induced Composition Gradients. Molten NaNO3 ‐ AgNO3 in Porous Yttria

Scanning electron microscopy with energy dispersive x‐ray fluorescence, and atomic absorption analysis, are applied to the chemical analysis of composition gradients in quenched binary mixtures of molten salts that have been electrolyzed between silver electrodes. The results demonstrate the merging of the diffusion layers from cathode and from anode, confirming the shapes of the composition profiles predicted from the ion flow analysis based on Faradaic, migrational, and diffusional flows. Effective diffusion coefficients for the electrolyte contained in a porous matrix are derived from measurements of the matrix porosity and of the cell conductance. The SEM/EDX analysis provides results with distance resolution of about 50 μm and precision of 5–6%. The methods are applicable to the analysis of composition gradients in molten salt battery electrolytes.