Cathodic Branch Research Articles

Behaviour of silicon electrodes in LiClKClPbCl 2 molten chlorides

Steady-state potentiostatic studies were carried out on n- and p-type Si in LiClKCl eutectic and in LiClKClPbCl 2 melts at 500°C. Mono-crystalline Si electrodes having the orientation (110) and the specific resistance of 0.01–200 Ω cm were investigated. The polarization curves obtained in the dark showed a current rectification effect. On the cathodic branch the sudden increasing of current was observed just at a few millivolts polarization, due to the silicon metallization. The cathodic currents have been found to be considerably higher than the anodic ones. The dependence of anodic current on semiconductor type, doping level and potential as well as PbCl 2 concentration in electrolyte is discussed.

Theoretical errors of linear polarization resistance and a method for reducing them

Three types of linear polarization resistance (LPR) may provide an approximate value of polarization resistance ( R p) for estimating the corrosion current density i corr: an anodic LPR, R a, a cathodic LPR, R c and a bi-directional LPR, R b. Theoretical errors of the LPS's depend upon two factors. One is related to the symmetric characteristic between anodic and cathodic branches of polarization curve and may be described by a parameter α = ( β II)/( β I), where β denotes the Tafel slopes and the subscripts indicate smaller and larger values respectively. The other factor is related to the polarization measurement of the LPR and can be expressed by (| ΔE |;)/(β I), where | ΔE | is the absolute value of polarization in one direction. (1) For the same value of (| ΔE |)/(β I), the largest theoretical error for R b will occur as a = 1 or α → ∞ whilst for R a or R c this will be as α → ∞ (2) The theoretical error of R b is always less than that of R a and R c under the same conditions except in the case of α → 1. When α = 1, the value of all three types of LPR are equal to each other. (3) The theoretical errors of LPR values increase with increasing of the value of (| ΔE |)/(β I), but in most cases the increasing rate for R b is far less than that for R a or R c. Based on the mathematical analysis, a method to reduce the theoretical error of R b was proposed. If two measurements are made in succession with the polarization values ± | ΔE 1 | and ± m |; ΔE 1 |;, where 1 < m ⩽ 2 and | ΔE 1 | ⩽ β I, and if the bi-directional LPR's thus measured are denoted by R b1 and R b2 respectively, then theoretical error of R p(approx.) as estimated by the equation R p (approx.) = (m 2−1)R b1 R b2 m 2R b2 −R b1 would be far less than that of R b1 and R b2.

Polarization behaviour of copper whisker crystal in acidic cupric sulphate solution

The polarization behaviour of copper whisker crystal was investigated in acidic cupric sulphate solution by a potential sweep method limited to 120 mV of overpotential. When the overpotential limit was above 130 mV, sweep experiments gave scattered results due to random nucleation. The apparent exchange current density on a whisker was around 10 −5 A/cm 2, three orders of magnitude less than that on a polycrystalline copper. The exchange current density is increased by the twisting of a whisker: it is proportional to the twist angle. The defect dependency of the exchange current is discussed in relation to a surface-diffusion model. Polarization curves at low overpotentials show asymmetry of the cathodic and anodic branch. The surface defects introduced by twisting appear to be four times as effective for anodic dissolution as for cathodic deposition.

Electrochemical formation and reduction of monomolecular oxide layers on (111) and (100) planes of gold single crystals

The anodic formation and cathodic reduction of monomolecular oxide layers was investigated by the potentiodynamic sweep method on (111) and (100) planes of gold single crystals. Only single peaks of oxide formation and reduction are observed in the anodic and cathodic curves on (100) as well as on (111). At dε/dt=10 mV s−1, the anodic oxide peak of (111) occurs at εH=1.7 V which is 0.2 V more positive than that of (100), εH≈1.5 V. The curves of oxide reduction coincide and show a peak near εH=1.2 V. The planes (111) and (100) are stable even during repeated cycles with a low sweep rate and at potentials, εH≤1.9 V, but a roughening and new peaks are observed at higher potentials, εH≥2.0 V. These disappear after some time by recrystallization. On both planes, the oxide formation can be described by the model of oxide growth derived earlier from measurements on polycrystalline material but the parameters differ and monolayer effects are observed. The mean oxide layer thickness in dependence on the potential differs on (111) and (100) at d≤2.5 Å but coincides approximately at d≥3.5 Å. The Tafel factors of initial anodic oxide formation at d=0 Å differ strongly, indicating a larger potential influence on (100) than on (111). At d≤2 Å the rate-determining step is a surface reaction; at d≥3 Å an ionic migration in the oxide may be rate-determining, too. The results on the cathodic branch agree on both planes and with previous measurements on polycrystalline gold electrodes. Hence, the structure of the oxide seems to be independent of the substrate orientation.

Electrochemical formation and reduction of monomolecular oxide layers on (111) and (100) planes of gold single crystals

Summary The anodic formation and cathodic reduction of monomolecular oxide layers was investigated by the potentiodynamic sweep method on (111) and (100) planes of gold single crystals. Only single peaks of oxide formation and reduction are observed in the anodic and cathodic curves on (100) as well as on (111). At de/dt=10 mV s−1, the anodic oxide peak of (111) occurs at eH=1.7 V which is 0.2 V more positive than that of (100), eH≈1.5 V. The curves of oxide reduction coincide and show a peak near eH=1.2 V. The planes (111) and (100) are stable even during repeated cycles with a low sweep rate and at potentials, eH≤1.9 V, but a roughening and new peaks are observed at higher potentials, eH≥2.0 V. These disappear after some time by recrystallization. On both planes, the oxide formation can be described by the model of oxide growth derived earlier from measurements on polycrystalline material but the parameters differ and monolayer effects are observed. The mean oxide layer thickness in dependence on the potential differs on (111) and (100) at d≤2.5 A but coincides approximately at d≥3.5 A. The Tafel factors of initial anodic oxide formation at d=0 A differ strongly, indicating a larger potential influence on (100) than on (111). At d≤2 A the rate-determining step is a surface reaction; at d≥3 A an ionic migration in the oxide may be rate-determining, too. The results on the cathodic branch agree on both planes and with previous measurements on polycrystalline gold electrodes. Hence, the structure of the oxide seems to be independent of the substrate orientation.