G. Fafilek

Study of grain boundary polarization by two-probe and four-probe impedance spectroscopy

Four-probe impedance spectra of polycrystalline cation conductors: NASICON, Na β-alumina and Ag β-alumina, were measured in the frequency range from 10 Hz to 100 kHz with the aid of high sensitivity, high input impedance preamplifiers. Dispersion associated with the grain boundary polarization was observed below room temperature. The frequency dependent part of the conductivity of grain interiors was seen in the four-probe spectra below 200 K. Two-probe impedance spectra were measured for samples with platinum electrodes over the frequency range 0.01 Hz to 10 MHz. The spectra were analyzed by nonlinear least-squares fitting of equivalent circuits. The circuit composed of two sections connected in series: (i) resistance R g parallel to constant phase element P gi , (ii) parallel combination of resistance R gb , constant phase element P gb and capacitance C gb , was found to be a good model of the dispersion seen in the four-probe spectra and was also part of the circuit used to simulate the two-probe spectra. The results obtained by the two-probe and four-probe methods for the resistance of grains, R gi , and total resistance, R t =R gi +R gh , are compared. The advantages and limitations of the two methods are discussed.

Instrumentation for ac four-probe measurements of large impedances

Abstract An ac four-probe set-up was constructed to measure impedance spectra between 1 Hz and 100 kHz on solid electrolytes up to impedance values of 10 GΩ. The influence of the system on the measurement was analysed for the previous and the new set-up. A solution to minimise the source of errors by pre-amplifiers for current and voltage signals is presented. A new sample holder was constructed for best results in conjunction with the new pre-amplifiers. The holder is designed for measurements on solid electrolytes over a wide temperature range. The performance of the improved set-up and the residual error sources are discussed. Measurements of impedance spectra on YSZ and BICUVOX.10 in a four-probe arrangement are given as an example.

Voltammetry on BIMEVOX.10 (ME=Cu, Ni, Zn, Mg) microsamples

Abstract High temperature cyclic voltammetry on microsamples was used for the investigation of the electrochemical stability of BIMEVOX.10 materials. The electrochemical measurements were performed on a three-electrode cell of the type: Pt|microsample|YSZ|Pt with the interface Pt|YSZ as a pseudo reference electrode. The experiments were performed in nitrogen containing a small quantity of oxygen. The cell consisted of a disc of yttria stabilized zirconia (YSZ) with platinum paste electrodes for the counter and the reference electrode. The working electrode was a small platinum plate which contacts the microsample and presses it onto the surface of the YSZ disc. In general, the stability of BIMEVOX.10 is determined by the stability of the main compound Bi 2 O 3 . None of the samples exhibited a substantially higher stability against complete reduction than BICUVOX.10. Nevertheless slight differences in the reduction processes at less negative potentials than the standard potential of Bi 2 O 3 have been observed. Redox processes in this potential region could be related to valence changes of vanadium from V 5+ to V 4+ and, at more negative potentials, from V 4+ to V 3+ .

Voltammetry on BICUVOX.10 microsamples

Abstract BICUVOX.10 (Bi 2 Cu 0.1 V 0.9 O 5.35 ) might be a very attractive material for use in oxygen sensors, fuel cells and oxygen separators. The electrochemical stability of BICUVOX.10 is an important factor for its future application. The electrochemical reduction and oxidation can be recognized very easily by the voltammetric technique. Our previous voltammetric measurements were made on 3-electrode cells like Pt,O 2 |BICUVOX.10|O 2 ,Pt using BICUVOX.10|0 2 ,Pt as reference electrode. This investigation showed a reduction of the material in the cathodic scan, followed by an oxidation peak in the anodic one. However, a quantitative elucidation of the reduction processes was not possible. The use of small samples in a cell of the arrangement Pt,Ar|sample|YSZ|Ar,Pt allows to distinguish between different reduction steps during the negative sweep. The reference electrode was a platinum point electrode applied to YSZ. Because of the small size of a sample it can be reduced to a large extent at temperatures above 600°C. We found that the sample started to be reduced irreversibly below −570 mV against the reversible oxygen electrode in 1 atm O 2 during the first negative sweep. Only part of the reduced sample can be reoxidised during the next positive sweep. The subsequent cycles showed reversible reduction and oxidation of the residual oxides.

Electrochemical stability studies by voltammetry on microsamples based on Bi26Mo10O69

Abstract The binary oxide Bi 26 Mo 10 O 69 is the parent compound of a new family of oxygen ion conductors with column type structure. High anion conductivity is observed along the [Bi 12 O 14 ] columns within the surrounding MoO 4 tetrahedra. High conductivity at intermediate temperatures is an important property but the electrochemical stability of the materials plays also an important role for future applications. Since bismuth-based electrolytes turned out to be not stable enough for an application in fuel cells, it is to be assumed that these materials are also easily decomposed. In this work, the decomposition limit was investigated by a voltammetric technique using very small samples. The results show a stability limit located approximately 100 mV more positive than in BIMEVOX. This is probably related to the reduction of Mo(VI) to Mo(IV). No change of oxygen stoichiometry was observed in the Bi 26 Mo 10 O 69 -based material in the course of a voltammogram before the primary reduction process occurs.

Comparative two- and four-probe impedance measurements on BICUVOX.10

Abstract In two-probe impedance measurements the interfacial impedance between metal electrode and ionic conductor contributes to the measured impedance. Therefore a calculation procedure which separates the bulk from the interfacial impedance must be applied. The separation procedure can be avoided by measuring the bulk conductivity in the four-probe arrangement. The impedance of polycrystalline samples and single crystals of the oxygen ion conductor BICUVOX.10 was measured in the a.c. two- and four-probe configuration. The frequency range was 0.1 to 1×10 6 Hz in the case of two-probe measurements and 0.1 to 1×10 5 Hz in the case of four-probe measurements. For single crystals, impedance spectra were taken in the direction parallel to the oxygen-conducting layers. The results of bulk conductivity obtained by four- and two-probe measurements were in agreement for polycrystalline samples and for single crystals. The origin of inductive loops visible for single crystals in the impedance spectra taken by the four-probe method is explained in a qualitative fashion as an interfacial effect at the voltage electrodes.

Automated device for electrochemical measurements and application to the system Pt/solid electrolyte

An automated device for voltammetric measurements at a prescribed set of constant temperatures was put together in our institute. It allows one to take steady-state current-potential curves in the potentiostatic or galvanostatic mode, to do cyclic voltammetry or coulometry at constant potential or to measure the potential-time dependence at a constant current. A special holder for the solid electrolyte was constructed. Voltammetric measurements on yttrium stabilized zirconia and of the new oxygen ion conductor BICUVOX.10 are presented as examples.

Developing of thin chromium-free multifunctional treatments on hot dip galvanized steel: strategy, implementation and experience

ZusammenfassungDa der Einsatz von Chrom(VI)-Verbindungen in der Automobil- und Elektronikindustrie weitgehend verboten ist, sind in den vergangenen Jahren viele verschiedene Chrom-freie Konversionsschichten entwickelt worden. In dieser Arbeit wurden verschiedene Cr-freie Phosphat- und Polycarboxylat basierte Konversionsschichten (Passivierungen) für die Anwendung als multifunktionale Korrosionsschutzschicht für feuerverzinktes Stahlband, mit Hilfe der Elektronenmikroskopie, EDX und Elektrochemie, charakterisiert. Die Korrosionsbeständigkeit im neutralen Salzsprühnebeltest der verschiedenen Konversionsschichten (Phosphat- und Polycarboxylatschichten) auf ZM- (ZnAlMg) und Z (Zn)-Substraten war ein weiteres Thema dieser Arbeit. Zusätzlich wurden die Konversionsschichten und deren Barriereschutzeigenschaften auf Zink und Zink Magnesium mit der Zyklovoltammetrie untersucht.SummarySince the use of hexavalent chromium in automotive and electronic equipment is largely prohibited, many different hexavalent chromium free conversion layers have been developed. In this work, different Cr-free phosphate and polycarboxylate based conversion coatings were characterized (with electron microscopy and other surface analytical methods) for the usage of a multifunctional property layer for hot dip galvanised steel strips. The different corrosion resistance of such conversion coatings on ZM (ZnAlMg) and Z (Zn) will be another topic of this paper. The corrosion protection properties of these phosphate based conversion coatings on Z and ZM were characterized in a standardized salt spray test. Finally the conversion layers and the barrier protection effect on Cr-free treated zinc and zinc magnesium was investigated by cyclovoltammetry.

On the volatilisation and decomposition of cyanide contaminations from gold mining

Cyanide leaching is the predominant process of gold extraction in large scale mining. Current initiatives for reducing the use of mercury in small scale and artisanal mining tend towards the cyanide technology as the only feasible alternative. Thus, the deliberate handling in consideration of the hazardous nature of cyanide compounds is an issue of particular importance. The hydrogen cyanide volatilised during the leaching process and from the tailings solutions after the gold extraction is reported to be destroyed by oxidation and photolysis from the surrounding atmosphere of gold mines and the sunlight. Cyanide solutions, drained into the surrounding waterbodies are stated to volatilise at a high rate, thus detoxifying them and releasing hydrogen cyanide to the air. In this study laboratory experiments and field tests were conducted to deliver basic data for the volatilisation and destruction of cyanide in the environment. In our laboratory tests we observed neither oxidation by the oxygen of air nor photolysis by UV-irradiation of cyanides after volatilisation from water. The whole amount of volatilised cyanide was found in the exhaust gas after absorption in a strong basic solution. Field experiments in Segovia (Colombia) could confirm these findings. Cyanide concentrations in a range 17 to 30mg/L were measured in a local creek. Hydrogen cyanide amounts of 5ppm were found in the atmosphere surrounding cyanide leaching facilities. With the findings of this study we want to point out that the concentrations of cyanide in the surrounding of cyanide leaching facilities exceed uncritical limits and a destruction via oxidation and photolysis is not detectable. These conclusions should result in initiatives to protect workers and the surrounding population of gold mines from contaminations of cyanide treatments.

Electrochemical properties of halogenated benzylidenehydrazino-pyrazoles in various imidazolium-based ionic liquids

The present paper deals with the anodic oxidation of various benzylidenehydrazino-pyrazoles in several room-temperature ionic liquids containing the imidazolium scaffold towards a more environmentally friendly route in obtaining the corresponding pyrazolo-triazoles. We have studied the voltammetric response of the latter substrates on a platinum disc electrode by varying the nature of the imidazolium cation to find the most suitable reaction medium. By changing the substituents attached to the para-position of the phenyl ring within the substrates, we intended to better understand the nature of the formed intermediate and to elucidate the mechanism of their electrode reaction.Graphical abstract