Ionic conductivity of the perovskites NaMgF3, KMgF3, KMgK3 and KZnF3 at high temperatures

We have carried out a study of the ionic conductivity of NaMgF₃, KMgF₃ and KZnF₃ up to temperatures close to the melting point. Our results, in contrast to previous reports in the literature, show no abnormal ionic conductivity at high temperatures. Care in interpretation of results is required because of surface electronic conduction.     

Electrical conductivity of Na2SO4(I)

The electrical conductivity of the solid phase Na₂SO₄(I) has been measured between the melting point at 884°C and the first order phase transition at about 240°C. The measurements have been performed using complex impedance measurements on pellet samples as well as on U-cells. The electrical conductivity is strongly dependent on sample at low temperatures and the activation energy ranges from 0.5 eV to 1.7 eV over the measured temperature range.     

Microscopic investigation of ionic conductivity in alkali metal salts-poly(ethylene oxide) adducts

We present conductivity, N.M.R. and D.S.C. measurements in two P(EO) complexes : P(EO) ₈LiCF₃SO₃ and P(EO) ₁₀NaI. From N.M.R. experiments, we deduce the respective amount of crystalline and elastomeric phases at all temperatures, as well as the salt concentration in these various phases. The elastomeric phase is shown to be responsible of the ionic conductivity at all temperatures, and to be very dilute (n ? 25 just above the pure P(EO) melting point. The high melting salt-rich complexes are found surstoechiometric (n ? 3.5). The various factors affecting the temperature dependence of the conductivity are discussed, as well as the kinetics problems.     

Effect of thermal history and characterization of plasticized,composite polymer electrolyte based on PEO and tetrapropylammonium iodide salt (Pr4N+I-)

The search for anionic conductors based on solid polymer electrolytes is important for the development of photo-electrochemical (PEC) solar cells due to their many favourable chemical and physical properties. Although solid polymer electrolytes have been extensively studied as cation, mainly lithium ion, conductors for applications in secondary batteries, their use as anionic conductors have not been studied in greater detail. In a previous paper we reported the application of a PEO based iodide ion conducting electrolyte in a PEC solar cell. This electrolyte had the composition PEO: Pr₄N⁺I^? = 9:1 with 50 wt.% ethylene carbonate (EC). In this work we have studied the effect of incorporating alumina filler on the properties of this electrolyte. The investigation was extended to electrical and dielectric measurements including high frequency impedance spectroscopy and thermal analysis.In the DSC themograms two endothermic peaks have been observed on heating, one of these peaks is attributed with the melting of the PEO crystallites, while the other peak with a melting temperature ~ 30 °C is attributed to the melting of the EC rich phase. The melting temperature of both these peaks shows a marked variation with alumina content in the electrolyte. The temperature dependence of the conductivity shows that there is an abrupt conductivity increase in the first heating run evidently due to the melting of the EC rich phase. High conductivity values are retained at lower temperatures in the second heating. Conductivity isotherms show the existence of two maxima, one at ~ 5% Al₂O₃ content and the other at ~ 15%. The occurrence of these two maxima has been explained in terms of the interactions caused by alumina grains, the crystallinity and melting of the PEO rich phase. As seen from latent heat of melting, the crystallinity of the electrolyte has reduced considerably during the first heating run. In contrast to the conductivity enhancement caused by ceramic fillers in PEO-based cation containing electrolytes, no conductivity enhancement has been observed in the present PEO based anionic conducting materials by adding alumina except at low temperatures.     

Electrolyte based on fluorinated cyclic quaternary ammonium ionic liquids

Ionic liquids, ILs, based on fluorinated pyrrolidinium and piperidinium ammonium cations and imide anion were prepared and characterized. The physicochemical and electrochemical properties of these ILs including melting point, glass transition and degradation temperatures, viscosity, ionic conductivity, and electrochemical stability were determined and compared to alkyl pyrrolidinium and piperidinium ILs. The incorporation of a CF₃ group instead of a CH₃ induces an increase of the IL viscosity, thus a conductivity decrease. However, good ionic conductivity is obtained with fluorinated pyrrolidinium IL. Cyclic amine ILs with propyl alkyl chain or fluorinated ammonium exhibit very high electrochemical stability toward oxidation. The effect of the addition of LiTFSI on the IL properties was studied with the same methodology.     

The melting mechanism in binary Pd0.25Ni0.75 nanoparticles: molecular dynamics simulations

The melting mechanism for Pd_0.25Ni_0.75 alloy nanoparticles (NPs) was investigated using molecular dynamics (MD) simulations with quantum Sutton-Chen many-body potentials. NPs of six different sizes ranging from 682 to 22,242 atoms were studied to observe the effect of size on the melting point. The melting temperatures of the NPs were estimated by following the changes in both the thermodynamic and structural quantities such as the total energy, heat capacity and Lindemann index. We also used a thermodynamics model to better estimate the melting point and to check the accuracy of MD simulations. We observed that the melting points of the NPs decreased as their sizes decreased. Although the MD simulations for the bulk system yielded higher melting temperatures because of the lack of a seed for the liquid phase, the melting temperatures determined for both the bulk material and the NPs are in good agreement with those predicted from the thermodynamics model. The melting mechanism proceeds in two steps: firstly, a liquid-like shell is formed in the outer regions of the NP with increasing temperature. The thickness of the liquid-like shell increases with increasing temperature until the shell reaches a critical thickness. Then, the entire Pd–Ni NP including core-related solid-like regions melts at once.     

Study of oxygen vacancy ordering in mullite at high temperatures

High temperature X-ray diffraction and quenching experiments of mullite single crystals with Al₂O₃:SiO₂ ratio 2:1 have been performed to investigate the stability of the oxygen vacancy ordering close to the melting point of mullite. The experiments show that the structure of mullite exhibits an extremely stable, temperature-independent incommensurate modulation. Inspection of satellite reflections at different temperatures leads to the conclusion that the ordering scheme of oxygen vacancies after the crystallization of mullite persists to the melting point and does not show any disordering effects. The experimental results are in agreement with former theoretical calculations using a statistical mechanics approach which yield the critical temperature T_c > 3000°C.     

Percolation of the liquid eutectic in random AlGe mixtures

If the metal fraction, X_i, in a metal-insulator mixture is less than the eutectic concentration, all of the metal will be converted to liquid eutectic at the melting point of the eutectic. The conducting fraction after melting is X_f > X_i near the percolation threshold, this increase in conducting friction can result in a large increase in sample conductivity, even if the conductivity of the eutectic is much less than the conductivity of the metal. Evidence for this behavior is presented from coevaporated AlGe films of random structure.     

MoSi2 laser cladding—a comparison between two experimental procedures: Mo–Si online combination and direct use of MoSi2

There are very strong interests in developing low density advanced material systems for service at temperatures up to 1300°C. These materials should mainly have moderate fracture toughness at low and intermediate temperatures and should exhibit oxidation resistant behaviour. The intermetallic compound, MoSi₂ has been considered to be an attractive candidate due to its melting point (2030°C) and excellent oxidation resistance at high temperatures. In this paper, we compare the results obtained with two different techniques for laser cladding, one using an online combination between Mo and Si powders, the second using direct injection of the MoSi₂ powder.     

Ionic and electronic conductivity in CaTi1−xFexO3−δ (x=0.1–0.3)

Electrical conductivity measurements on CaTi_1?xFe_xO_3?δ (x=0.1, 0.2, 0.3) were performed on polycrystalline pressed and sintered tablets using the van der Pauw four point method in controlled atmospheres. The results were interpreted to reflect n-type, ionic and p-type conductivity at different oxygen partial pressures. An increasing iron content increases the number of oxygen vacancies and increases the ionic conductivity at high temperatures, but also increases the tendency of ordering, which suppresses the ionic conductivity at more moderate temperatures. These findings are in accordance with the phase diagram of the system CaTiO₃-CaFeO_2.5 based on X-ray and Mössbauer studies.     

On viscous mechanism for surface diffusion at high temperatures (T/Tm > 0.75) due to formation of a 2D dense fluid on metallic surfaces

Experimental determinations of temperature dependence of surface self-diffusion coefficient of several metals exhibit a strong increase in D_s values and in activation energy for temperatures near the melting point T_m. This variation is illustrated by a bending of the Arrhenius plot of surface self-diffusion coefficients of tungsten, which are obtained experimentally by tip profile variation technique. For T/T_m < 0.75 the apparent activation energy for W is 2.85 eV and the pre-exponential term is equal to 0.24 cm²/s, while for T/T_m > 0.75 we have respectively 5.57 eV and 1.08 × 10⁴ cm²/s. To account for these unexpected variations in the activation energy and diffusivities, the hypothesis that the surface mass transport mechanism changes from individual atomic jumps at low temperatures towards a cooperative motion at temperatures near the bulk melting point, namely a viscous mechanism, is proposed. This model is based on the postulation of the formation of a 2D dense fluid on the metallic surfaces about 75% of the bulk melting temperature. Discussions of existing models on surface diffusion proposed by Rhead, by Bonzel, or by Tsong are given, and a technique to characterize surface viscosity of a 2D dense fluid is suggested.     

Temperature-dependent absorptance of painted aluminum, stainless steel 304, and titanium for 1.07 μm and 10.6 μm laser beams

We measured the temperature-dependent absorptance of metals (Al, Ti, SS304) for continuous beams from 1.07 μm fiber laser and 10.6 μm CO₂ laser using power sensors and infrared (IR) pyrometers. The absorptance measurements were repeated for metals with three different paint coatings. For measurements at elevated temperatures up to the melting point, integrating sphere is not practical since high temperature radiation from a heated target disturbs weak output from the sphere considerably. Our results provide how each metal, whether coated or uncoated, absorbs the infrared beams as temperature is elevated to a melting point. A polynomial approximation to the measured absorptance of each target is provided for modeling of the laser-metal interaction at elevated temperatures.     

Dielectric parameters of mesoporous sieves filled with NaNO<Subscript>2</Subscript>

This paper reports on a dielectric study of MCM-41 molecular sieves with cellular channels of different sizes filled with the NaNO₂ ferroelectric. The temperature dependences of the permittivity and electrical conductivity of sodium nitrite in cellular channels are calculated from experimental data on the permittivity and electrical conductivity of the composite. The calculations are performed using the relationships obtained for the hexagonal matrix with parallel cylindrical inclusions within pores. The observed increase in the conductivity of sodium nitrite in confined geometry at high temperatures is attributed to partial melting. It is shown that the increase in the permittivity of the composite is caused by Maxwell-Wagner relaxation processes.     

Surface effect on the coalescence of Pt clusters: A molecular dynamics study

We have performed molecular dynamics simulations for Pt_N + Pt_N → Pt_2N (N = 147, 324, 500,792), to investigate the effect of size and substrate on coalescence temperature. Our simulations show that platinum nanoclusters coalesce at the temperatures lower than the cluster melting point. The difference between coalescence and melting temperatures decreases with the increase in cluster size and presence of substrate. These thermal behaviors affect catalytical properties of nanoclusters and the substrate, as an environment, has major effect on activity of metal nanoclusters.     

Molecular dynamics simulation of a graphite-supported copper nanocluster: thermodynamic properties and gas adsorption

Molecular dynamics simulations were conducted for a cubic Cu cluster supported on a graphite bilayer. The Sutten–Chen and Lennard–Jones potentials were used for metal–metal and metal–graphite interactions, respectively. Heating and cooling processes were performed by NVT simulations at different temperatures in the range 200 to 1800?K. The melting point was identified on the basis of caloric and heat capacity curves. The calculated melting point was 770?K, far below the bulk melting point of crystalline copper. Several phenomena such as the appearance of a hysteresis (irreversibility) in caloric curves, surface melting, and cluster-induced surface wetting were justified from the results. The simulation of cluster in the presence of gas atmosphere showed that the CO gas is adsorbed more than H₂ and it has a greater impact on the cluster's structure.     

Electrical properties of lithiated boron oxide fast-ion conducting glasses

Lithiated boron oxide glasses of the family xLi₂O-B₂O₃, with x ranging from 1.0 to 4.5, were prepared by melting at high temperatures and shaped in the form of slabs. The glasses’ properties were evaluated by Complex Impedance Spectroscopy and Scanning Electron Microscopy techniques. The variation of lithium-ion conductivity at room temperature as a function of the glasses’ lithium content is found to exhibit a maximum at a molar ratio of x=3.5. This maximum in the ionic conductivity is correlated to the formation of crystalline regions in the vitreous structure of the material as the lithium concentration is increased. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15–21, 2002.     

Study of dc ionic conductivity in single crystals of RbNO3

dc ionic conductivity has been studied on single crystals of RbNO₃ grown by slow evaporation method. The range of temperature covered is from room temperature to its melting point atmospheric pressure. An attempt is made to correlate the known phase transitions with the variation of ionic conductivity with temperature.     

X-ray study of molecular oxygen adsorbed on graphite

We have performed a detailed X-ray diffraction study of O₂ adsorbed on UCAR-ZYX and Le Carbon Lorraine vermicular exfoliated graphite between 15 and 50 K. At least four phases of physisorbed oxygen are found. The monolayer δ phase consists of a centered parallelogram lattice, with the molecular axes parallel to the graphite surface. The data are consistent with a triple point at 26 K. The melting transition at a coverage of one monolayer appears to be first order. At higher coverages the molecules undergo a lying-down to standing-up transition; the higher coverage ζ phase froms an approximately triangular lattice with the molecular axes perpendicular to the graphite surface. Satellite peaks around the (1, 0) Bragg peak indicate, however, that this cannot be a simple triangular lattice; possible explanations include successively incommensurate layers or a sinusoidal density modulation. For coverages in the two-layer region the ζ phase modulation peaks disappear at 37 K, and at 40 K the adsorbed oxygen appears to undergo a first order melting transition into a fluid phase. With increasing coverage, the 2D X-ray diffraction profiles and phase boundaries do not connect smoothly onto those of the 3D α and β phases. At low temperatures (T < 30 K) the ζ phase always coexists with bulk crystallites; for temperatures near the 2D melting transition the 3D peaks are not observable. These data, taken together with the heat capacity results, suggest a wetting transition with only the bilayer lamellar phase or bulk O₂ being stable at low temperatures.     

Effect of melt compounding temperature on dielectric relaxation and ionic conduction in PEO–NaClO<Subscript>4</Subscript>–MMT nanocomposite electrolytes

Polymer nanocomposite electrolytes (PNCEs) of poly(ethylene oxide) and sodium perchlorate monohydrate complexes with montmorillonite (MMT) clay up to 20 wt.% MMT concentration of poly(ethylene oxide) (PEO) are synthesized by melt compounding technique at melting temperature of PEO (∼70 °C) and NaClO₄ monohydrate (∼140 °C). Complex dielectric function, electric modulus, alternating current (ac) electrical conductivity, and impedance properties of these PNCEs films are investigated in the frequency range 20 Hz to 1 MHz at ambient temperature. The direct current conductivity of these materials was determined by fitting the frequency-dependent ac conductivity spectra to the Jonscher power law. The PNCEs films synthesized at melting temperature of NaClO₄ monohydrate have conductivity values lower than that of synthesized at PEO melting temperature. The complex impedance plane plots of these PNCEs films have a semicircular arc in upper frequency region corresponding to the bulk material properties and are followed by a spike in the lower frequency range owing to the electrode polarization phenomena. Relaxation times of electrode polarization and ionic conduction relaxation processes are determined from the frequency values corresponding to peaks in loss tangent and electric modulus loss spectra, respectively. A correlation is observed between the ionic conductivity and dielectric relaxation processes in the investigated PNCEs materials of varying MMT clay concentration. The scaled ac conductivity spectra of these PNCEs materials also obey the ac universality law.