Molten ZnCl2 Research Articles

Brillouin scattering study of molten zinc chloride.

Polarized and depolarized Brillouin scattering experiments on molten ZnCl2 were performed between 300 and 600 degrees C in different geometries. VV spectra measured in backscattering and small angle scattering were analyzed with conventional viscoelastic theory using either a Debye or a Cole-Davidson model for the memory function. We also analyzed in the same way the temperature dependence of the transverse Brillouin lines detected in a 90 degrees VH geometry. We show that the Cole-Davidson memory function yields a consistent interpretation of all the spectra. The resulting shear and longitudinal relaxation times are equal within their error bars, and are about 2.5 times smaller than the alpha relaxation time previously determined. The static shear viscosity values deduced from the analysis of the propagating transverse waves agree, at all temperatures, with the measured viscosity values.

On the partial structure factors of molten zinc chloride

The partial structure factors (PSF) of molten ZnCl2 at 600 K have been known since 1981 due to the neutron diffraction experiments by Biggin and Enderby (S. Biggin and J. E. Enderby, J. Phys. C, 1981, 14, 3129). It is shown in this article that the set of PSF derived from this experiment is not consistent with new X-ray diffraction data with much better statistical accuracy. A new set of PSF is derived, which corrects this deficiency. The first diffraction peak is not dominated by the cation–cation-PSF and the set of PSF shows similarities to the PSF of glassy GeO2 and SiO2 determined recently. The derived PSF are shown to be insensitive to whether ionic or atomic form-factors are used.

Induced-dipole contributions to the conductivity and dielectric response of molten ZnCl2

A molecular dynamics simulation of molten ZnCl2 with a realistic interionic potential is used to evaluate the contribution of interaction-induced dipoles to the dielectric response, or equivalently, to the conductivity. The induced dipoles are included self-consistently in the interionic potential. The contribution is found to be significant across the accessible range of frequencies, modifying various features of the spectrum ordinarily attributed to elementary translations of the ionic charges, and markedly improving agreement with experimental spectra.

Ionic charge transport in strongly structured molten salts

Data on the DC ionic conductivity for strongly structured molten halides of divalent and trivalent metals near freezing are interpreted as mainly reflecting charge transport by the halogen ions. On this assumption the Nernst–Einstein relation allows an estimate of the translational diffusion coefficient D tr of the halogen. In at least one case (molten ZnCl 2) D tr is much smaller than the measured diffusion coefficient, pointing to substantial diffusion via neutral units. The values of D tr estimated from the Nernst–Einstein relation are analyzed on the basis of a model involving two parameters, i.e. a bond-stretching frequency ω and an average waiting time τ. With the help of Raman scattering data for ω, the values of τ are evaluated and found to mostly lie in the range 0.02–0.3 ps for a vast class of materials.

Monte Carlo and reverse Monte Carlo simulations on molten zinc chloride

Monte Carlo and reverse Monte Carlo simulations on molten ZnCl2 were performed on the basis of the total atom pair correlation functions, recently determined with X-ray diffraction experiments at T=623 and 873 K. A new pair-wise additive potential model is presented that is based on an intermediate-range oscillating function for the Zn–Zn interaction in combination with a modified Coulomb potential. This potential mimics the effect of induced anion polarization, which has been found as to be the reason for the highly directional interactions between the cations and anions in the melt. With this simple potential model, the experimentally determined total atom pair correlation functions and the total structure factor, the prepeak included, are well reproduced. The analysis of the configurations, generated with the Monte Carlo method, confirms the existence of a network structure with rings and chains of small and medium size consistent with corner connected [ZnCl4] units of high tetrahedricity. In contrast, the reverse Monte Carlo method generates more disordered structures and is obviously not able to give a correct description of the ion arrangement in molten ZnCl2.

The structure of molten ZnCl 2

High energy photon diffraction measurements on molten ZnCl 2 have been carried out in the temperature range 623–853 K. The momentum transfer range covered is 0.15 Å −1< Q<21.6 Å −1. The sample is investigated in transmission geometry. The data are in good agreement with neutron diffraction measurements. A discrepancy at smaller Q values is to be noted, however, with earlier studies using conventional Mo–K α X-rays in reflection geometry. By combining neutron and high energy photon results, the partial distribution functions can be separated to some extent. Almost no change of the first Zn–Cl distance is observed with increasing temperature, whereas the distribution of the first shell Cl–Cl distances increases in width and the first shell Zn–Zn distance decreases.

High temperature phase segregation of a new host for Er3+ upconversion: Cs3Tl2Cl9

Cs3Tl2Cl9 is one of the few air stable low-phonon host lattices of interest to Er3+ upconversion. Solution growth at ambient temperature demonstrated that water and a number of other molecular liquids do not yield Er3+doped crystals. Optical and differential scanning calorimetry measurements revealed a phase segregation at 310 °C which is reversible in the presence of 1 atm Cl2 . Growth from molten salt solutions or gas phase deposition techniques is hence restricted to a deposition temperature of less than 300 °C. Preliminary results show that molten ZnCl2 may be used as a flux to obtain lanthanidedoped single crystals or epitaxial layers.

Chemical and electrochemical behaviour of copper ions in the ZnCl 2-2NaCl mixture at 450 °C

To investigate the possibility of selective chlorination to separate copper traces from zinc matrixes, we have determined redox and oxoacidobasic properties of the copper in the molten ZnCl 2-2NaCl. In this way, the stability of copper chlorides and oxides as well as the electrochemical behaviour of Cu(I) have been studied by using voltammetric, chronopotentiometric, chronoamperometric and potentiometric techniques. The experiments were performed in a controlled argon atmosphere cell with safety features. Copper oxidation states I and II have been shown to exist in the mixture and the stability of metallic copper has been confirmed. The standard potential of the redox couple Cu(II) Cu(I) and Cu(I) Cu(0) have been determined by voltammetry ( E Cu(II) Cu(I) 0 = − 0.135 ± 0.016 V vs Cl 2 Cl − ) and by potentiometry ( E Cu(I) Cu(0) 0 = − 1.129 ± 0.030 V vs Cl 2 Cl − ), respectively, finding that Cu(II) ion can oxidize, under certain conditions, the chloride ion of the melt. Moreover, the stability of both Cu(I) and Cu(II) oxides was investigated and the solubility product of CuO was determined, whereas Cu 2O was found to be a hard oxide donor. These results have allowed the construction of E- pO 2− equilibrium diagram. On the other hand, the kinetics of copper electrodeposition and electrocrystallization at a tungsten electrode were studied, and it was demonstrated that the process is quasi-reversible. In the same way, the Cu(II) Cu(I) exchange was found to be quasi-reversible. The values of the kinetic parameters, K 0 and α, were obtained for both reactions. Mass transport towards the electrode is a simple diffusion process, and the diffusion coefficient has been calculated. Chronoamperometric studies indicated instantaneous and three dimensional nucleation and crystal growth of copper.

Structural studies of some chloride metls with EXAFS

The structural parameters as well as the radial distribution functions of the nearest-neighbor atoms around Zn or Rb atoms in molten ZnCl 2, RbCl and Rb 2ZnCl 4 are obtained with EXAFS measurement. The tetrahedra consisting of four Cl atoms around Zn are found to be dominant and stable in the molten ZnCl 2. The skewed behavior of radial distribution function for RbCl in the molten RbCl is also found, while the local structures around Rb can be regarded as a persistence of the solid state but with large short-range disorder. In the molten Rb 2ZnCl 4, the network linked by ZnCl 4 groups is not found and the nearest-neighbor structures around Rb atoms are more disordered than in molten RbCl. The results of molecular dynamic simulations are coincident well with that of the EXAFS.

EXAFS studies of molten ZnCl2, RbCl and Rb2ZnCl4

The local structures of molten ZnCl2, RbCl and Rb2ZnCl4 are measured with the EXAFS technique. Several models are used to analyse the data, and some structural parameters as well as the radial distribution functions of the nearest-neighbour atoms around Zn or Rb atoms are obtained. The tetrahedra consisting of four Cl atoms around Zn are found to be dominant and stable in molten ZnCl2. The skewed behaviour of the radial distribution function for Rb-Cl in molten RbCl is also found, while the local structures around Rb can be regarded as a persistence of the solid state but with strong short-range disorder. In molten Rb2ZnCl4, a network linked by Zn Cl4 groups is not found and the nearest-neighbour structures around Rb atoms are more disordered than in molten RbCl. This has also confirmed that the EXAFS method can provide reliable information about the nearest-neighbour atom-atom coordination for some molten salts, which is helpful in specifying the painwise interaction potential.

Real space manifestation of the first sharp diffraction peak in the structure factor of liquid and glassy materials

The problem of those discernible features of the intermediate range order (IRO) which can be attributed to the first sharp diffraction peak (FSDP) observed in the structure factor of many liquid and glassy materials is approached by treating this peak as a distinct feature. It is found, by considering the measured partial structure factors, S αβ ( k ), for molten ZnCl 2 , GeSe 2 , MgCl 2 , NiBr 2 and Nil 2 and the measured total structure factors, F ( k ), for glassy SiO 2 , PS 4 and liquid CCl 4 , that the propensity of the FSDP to have a prominent effect on the underlying features of the IRO depends noticeably on the system type. Specifically, the FSDP confers a marked oscillatory character of periodicity 2π/ k 1 (where k 1 is the FSDP position) on the IRO when the local structural units, which give rise to the density fluctuations on the IRO scale, exist as stable entities for a timescale τ ≫ 5 × 10 -12 s. The FSDP therefore accounts for the discernible features of the underlying IRO for the viscous glass forming liquids ZnCl 2 and GeSe 2 , for the glasses SiO 2 and PS 4 , and for the molecular liquid CCl 4 . The influence of the FSDP on the IRO is less pronounced for molten MgCl 2 and is negligible for molten NiBr 2 and Nil 2 , both of which have a high cation mobility which leads to a relative instability of the Ni 2+ centred structural units. The effect on the FSDP of temperature and pressure are briefly considered as are the development of the FSDP in molten ZnX 2 (when X is changed from Cl to I to Br) and the minimum size of r -space model which is required if the FSDP is to be accurately predicted.

Rayleigh‐Brillouin Scattering of Molten ZnCl2

AbstractThe Rayleigh‐Brillouin scattering experiment has been carried out for molten ZnCl2. The complete profiles of Rayleigh‐Brillouin spectra were obtained at scattering angles of 90° and 140°, and at temperatures ranging from 550 to 1000 K by the use of the carefully purified samples. The remarkable change in the shape of the spectrum with temperature variation has been observed and has been concluded to be due to a structural relaxation in the melt. The spectra have been analyzed by the viscoelastic theory on the assumption of a single relaxation. The sound velocities at the limiting low and high frequencies and the relaxation time have been determined.

Simultaneous liquefaction of a subbituminous coal and upgrading of bitumen with molten ZnCl 2-based catalysts

Molten halide catalysts have been found to have excellent cracking activity towards coal and bitumen-related structures as has been reported in the literature. Upgrading experiments have been carried out in a batch autoclave to study the effectiveness of molten halide catalysts consisting of ZnCl 2 and its mixtures with MoCl 5, on simultaneous liquefaction of a subbituminous coal and upgrading of Athabasca bitumen. Two methods of catalyst addition to the reaction slurry were compared. Impregnation of the coal with a catalyst-saturated methanol solution was found to be more effective than direct addition of the pulverized catalyst. The effects of different variables such as temperature, reaction time, catalyst quantity and catalyst formulations were studied. The optimum temperature and reaction time were found to be 400°C and 2 h respectively. Under these conditions, a net conversion of 75% of the d.a.f. (dry ash-free) coal was achieved.

Viscoelastic Properties of Molten ZnCl2-MCl (M: Na, K) Binary Systems.

In order to clarify the viscoelastic properties of ZnCl2-MCl (M: Na, K) binary melts, hypersonic velocities have been measured in the temperature range covering about 150 K above the liquidus temperatures of the melts over the composition range of 0 to 50 mol% MCl by means of the Brillouin scattering method. Absorption coefficients of ultrasound were measured for molten ZnCl2 at the frequency range of 5-55 MHz. The hypersonic velocity of ZnCl2 decreases linearly with increasing temperature at the high and the low temperature regions, but at the intermediate temperature region, shows a curvilinear temperature dependence, indicating dispersion of sound propagation.The dispersion behavior observed is well described in terms of the single relaxation theory. It was found that the relaxation frequency of the binary melts increases steeply with an addition of MCl. This is considered to be due to the depolymerization of the network structures in the melts.

The structure of molten and glassy 2:1 binary systems: an approach using the Bhatia—Thornton formalism

A systematic analysis of those liquid binary 2:1 systems (denoted MX 2 ), for which experimental partial structure factors are available from the isotopic substitution method in neutron diffraction, is made using the Bhatia-Thornton (BT) formalism.Particular attention is paid to the origin of the first sharp diffraction peak (FSDP ), which occurs in the measured diffraction patterns for some of the MX 2 systems, since it appears, from recent studies, that this feature is a signature of directional bonding. It is found that FSDPS can occur in all three BT partial structure factors S xB (k). A FSDP feature in the concentration-concentration partial structure factor S cc (k) is not, however, pronounced except in the case of MgCl 2 and the glass forming network melts ZnCl 2 and GeSe 2 . To the extent that these systems can be regarded as ionic melts a FSDP in S cc (k) implies a non-uniformity in the charge distribution on the scale of the intermediate-range order (IRO). The structure of molten GeSe 2 is compared with the structures of molten ZnCl 2 , glassy GeS 2 and glassy Si0 2 . Although the GeSe 2 and ZnCl 2 melts have different short-range order, there are similarities in the observed IRO which can be attributed to the arrangement of the electropositive species M. The essential features of the measured total structure factor for glassy GeS 2 can be reproduced by using the molten GeSe 2 S zB (k). This result lends support to the notion that the S zB (k) for liquid GeSe 2 (and ZnCl 2 ) are characteristic of both the liquid and glassy states of other network glass forming systems. The structures of molten GeSe 2 (or ZnCl 2 ) and glassy Si0 2 are, however, found to be different. The observed discrepancies are largest in the region of the FSDP which signifies pronounced differences in the nature of the IRO for these systems.

Pyrolysis of lignins in molten salt media

Two kinds of lignins (kraft and solvolysis lignins) were pyrolyzed using mixtures of molten ZnCl 2 and KCl (molar ratios 3/7 and 7/6) at 500-800°C under atmospheric pressure. The yields of gaseous products and of phenolic compounds were measured. The variations of gaseous product yields with temperature were similar in pattern, irrespective of the lignin type. A higher yield of H 2 implies that the H 2 O forming reaction was partially retarded. It was also found that the mixture of molten ZnCl 2 and KCl with molar ratio 7/6 had a high selectivity to cresols.

Structure of the Molten ZnCl&lt;SUB&gt;2&lt;/SUB&gt; near Cathode

In order to investigate the structure of an ideally polarized electrical double layer of the molten ZnCl 2 (mp 591 K), a molecular dynamics simulation between two walls, one of which is negatively charged, was carried out at 600 K. Capacitance of the double layer developed at the cathode shows linear dependence on applied voltage, i.e. a simple Helmholtz type double layer, which agrees with experimental results. Computational and experimental capacitances agree well

Tetrahedral coordination of Zn ions in molten zinc halides

Measurements of the scattering of thermal neutrons from natural samples of molten ZnCl2, ZnBr2, and ZnI2 are reported. The experiments were undertaken on the liquids and amorphous materials diffractometer of the ISIS spallation source at the Rutherford–Appleton laboratory. The data reveal some shortcomings in earlier studies of ZnCl2 but confirm a structural model in which the small Zn2+ ions occupy tetrahedrally coordinated sites in a closely packed anion structure, with strong intermediate range ordering. Features arising from changes in the anion species are identified and temperature-dependent effects are resolved in ZnCl2.

The influence of pressure on the electrical conductivity of molten zinc chloride and its Mixtures with potassium chloride

Abstract The electrical conductivities of molten ZnCl2 and its mixtures with KCl were measured as functions of pressure, temperature, and composition. The measuremkents were performed in an internally heated pressure vessel in which the melts were contained in open quartz glass cells. The addition of KCl to molten ZnCl2 causes a large increase of the conductivity at all pressures and temperatures. With increasing pressure the conductivity increases in pure molten ZnCl2 and in mixtures rich in ZnCl2 and decreases in mixtures with more than 30 mol% of KCl.

Models and structure predictions in computer simulation of molten ZnCl2

We report molecular dynamics results for pair radial distribution functions, coordination numbers, total energy and internal pressure in molten ZnCl2. The assumed pair potential is described by Coulomb interaction added to short-range overlap repulsion according to the conventional Born-Mayer description of pair interactions. The effects of a distance-dependent dielectric screening and/or the non-additivity of ionic radii in such a pair potential are investigated in a large set of models. The hardness parameters that describe the exponential decay of the short-range repulsions were adjusted in order to optimize the zinc-zinc pair correlation function. The obtained structural properties agree on a whole with the neutron scattering data better than previous molecular dynamics results.