Ionic,electronic and ion-diffusion controlled relaxation processes in CaF2, BaF2 and LiBaF3 crystals

Abstract

The ionic, electronic and anion-diffusion controlled thermally stimulated relaxation (TSR) processes at 80—700 K in CaF₂ BaF₂ and LiBaF₃ crystals (X-ray irradiated or non-irradiated) have been investigated by means of ionic conductivity, ionic thermally stimulated (TS) depolarization current (TSDC); as well as current (TSC), luminescence (TSL) and bleaching (TSB) techniques. Above 250—290 K broad and overlapping anion TSDC peaks and correlated TSB stages are detected. The TSB kinetics is initiated and controlled by anion detrapping and interaction with the localized charges, i.e., the anion-diffusion controlled TSR processes take place in fluorides. The TSL and TSC data for LiBaF₃ indicate that the lifetime and drift of electrons at 80—250 K is very small because of deep retrapping. The main TSL peaks at 132K, 170K and 220 K are caused by V_k center detrapping and hole-diffusion controlled tunnel recombination within pairs like {D_n e^?…V_k }.     

Electron paramagnetic resonance spectroscopy of Cr 3+ in hexagonal Cs 2 NaGaF 6 crystals

Powder samples of hydrothermally grown Cr 3+ -doped Cs 2 NaGaF 6 crystals have been investigated with electron paramagnetic resonance spectroscopy at X - (9.5 v GHz) and Q -band (34 v GHz). Analysis of the spectra clearly demonstrates that there are two distinct Cr 3+ centres in the Cs 2 NaGaF 6 crystal, having nearly identical g factors, but differing largely from the viewpoint of their zero field splitting. By using the 53 Cr hyperfine spectra observed with electron nuclear double resonance spectroscopy, it is deduced that these centres have opposite signs for the zero field splitting. The spectroscopic properties of the Cr 3+ centres in the isostructural Cs 2 NaGaF 6 and Cs 2 NaAlF 6 crystals are compared and discussed.     

Electric conductivity and relaxation in ZnF2–AlF3–PbF2–LiF glasses

The electric conductivity of ZnF₂–AlF₃–PbF₂–LiF glasses has been studied in the frequency range 10 Hz–2 MHz and in the temperature range from 300 K to just below the glass transition temperature. The conductivity decreases with the increase in the LiF content in the composition, which results from the trapping of F⁻ ions by Li⁺ ions. Small values of the stretching exponent β are observed for the present glasses. The value of the decoupling index decreases with an increase in LiF content, consistent with the composition dependence of the conductivity.     

Preparation of carbon-aerogel polypyrrole composite for desalination by hybrid capacitive desalination method

Capacitive Deionization (CDI) is an emerging technology with great potential applications. Most researchers view it as a viable water treatment alternative to reverse osmosis. This research reports the preparation and application of a carbon aerogel polypyrrole (CA-PPy) composite for the desalination of NaCl solution by the hybrid CDI method. The carbon aerogel (CA) was prepared from a Resorcinol / Formaldehyde precursor by the sol–gel method. The aerogel obtained from the sol–gel was then pyrolysed in a tube furnace to form CA. Polypyrrole (PPy) was prepared by the Oxidative chemical polymerisation of pyrrole, ferric chloride hexahydrate (oxidant), and sodium dodecyl sulfate (dopant). A composite of CA and PPy was then prepared and used to modify carbon electrodes. The CA-PPy composite was characterised to verify its composition, morphology, thermal properties, and functional groups. The electrochemical properties of the material were determined by Cyclic voltammetry (CV) and Electrochemical impedance spectroscopy (EIS) tests. The electrochemical tests were done using a GAMRY potentiostat electrochemical workstation, a 1.0 M KCl was used as the electrolyte, and the applied potential window was (-0.2 to + 0.6) V for the CV test. The EIS test was done with the same concentration of KCl electrolyte at an applied potential of 0.22 V and at a frequency range of (0.1 – 100, 000) Hz. The optimal specific capacitance of the CA is 115F/g, and that of the composite is 360.1F/g, they were both obtained at a scan rate of 5 mV/s. The CDI desalination study of the CA-PPy composite showed a salt adsorption capacity (SAC) of 10.10 mg/g (300 mg/L NaCl solution) – 15.7 mg/g (800 mg/L NaCl solution) at 1.2 V applied voltage. The salt recovery efficiency of the electrode material in the 300 mg/L solution is 27 %, in the 500 mg/L solution, it is 20.12 %, and in the 800 mg/L solution, it is 15.41 %. The electrode material also showed good electrochemical stability after nine cycles of ion adsorption/desorption study.