Milenko V. Šušić

Thermic investigations of hexagonal hydrogen bronze, HxWO3

The thermic-kinetic behaviour of hydrogen tungsten bronze, HxWO3, was investigated by differential scanning calorimetry and X-ray analysis. It is shown that the thermal decomposition of bronze into hydrogen and WO3, is exothermic. Further, it is shown that the hexagonal WO3 system is exothermally transformed (irreversibly) to the monoclinic WO3 system. Activation energies, frequency factors and rate constants of the corresponding processes are determined. It is shown that the hexagonal WO3 system adsorbs hydrogen at approximately 480 K, and desorbs at room temperature in air or an argon-nitrogen atmosphere. It is also shown that HxWO3 is unstable and that it decomposes with time, forming brown WO3 which gives no yellow modification during the WO3 phase transformation. The newly formed modification adsorbs hydrogen without changing colour.

Multiple hydriding/dehydriding of Zr1.02Ni0.98 alloy

Abstract The hydriding kinetics and hydrogen absorption capacity of Zr 1.02 Ni 0.98 alloy powder were investigated in a temperature range of 150-250C. An isothermal volumetric method within a pressure range of 0-1bar was used. As expected, the temperature increase causes both an increase of the hydriding rate and a shift in the equilibrium of the Zr 1.02 Ni 0.98 + x /2H 2 -Zr 1.02 N 0.98 H x reaction towards the left. Performing multiple hydriding/dehydriding cycles, an increase in hydriding rate accompanied by a decrease in maximal hydriding capacity was observed after each subsequent hydriding cycles of the same sample. Using X-ray diffractometry, this was shown to be a consequence of an irreversible destruction of the alloy.

Kinetic-thermal processes of hydrogen sorption on Pd/WO3 and Pd/MoO3 bronze

Kinetic-thermal investigations of the hydrogen sorption process in the temperature range from ambient to 720 K on powder samples of Pd/WO3 and Pd/MoO3 show that it takes place according to the “hydrogen spillover” mechanism, during which the corresponding hydrogen bronzes are formed. Hydrogen sorption changes the structure of the Pd/WO3 bronze, while it breaks the structure of the Pd/MoO3 bronze producing the amorphous state on heating to 570 K. The hydrogen bronze Pd/HxWO3, heated in oxygen, decomposes at around 830 K, resuming the original form of the yellow Pd/WO3 bronze. A repeated cyclic heating of Pd/HxMoO3 bronze in hydrogen and oxygen in turn (thermogravimetric analysis and differential scanning calorimetry methods) shows that the sorbed hydrogen reacts violently with oxygen and to form water. In the opposite way, hydrogen sorption on the sample after heating in oxygen also proceeds violently, producing hydrogen bronze after the initial formation of water with oxygen. When heated in oxygen after hydrogen up to 720K, the sample is oxidized to the maximum extent, returning to the original grey colour and the crystal state. Further heating in hydrogen produces a hydrogen bronze of dark blue colour, while the structure is decomposed. The kinetic processes were investigated and kinetic and thermal parameters were determined. The change in structure is also demonstrated by X-ray diffractometry and electron microscopy.

Electric and electrochemical properties of solid LiH2PO4

Abstract Electrical and electrochemical properties of solid LiH 2 PO 4 conductor were investigated in the temperature range from room temperature to 373 K. It was found that high conductivity throughout the temperature range, with activation energy 17.23 kJ/mol, originates from the movement of hydrogen ions (protons). The movement of protons in the correlation with phosphate groups rotation was considered. The slopes of Tafel lines and exchange current densities both for cathodic hydrogen and anodic oxygen evolution were determined (by means of usual electrochemical kinetic methods) at various temperatures. The energy of activation at the equilibrium potentials both for the cathodic and the anodic processes have been assessed to be 17.23 kJ/mol (0.18 eV) and 2.9 kJ/mol (0.03 eV), respectively.

The electrochemical properties of cadmium forms of zeolite of type A

Abstract The electrochemical behaviour of pressed cadmium forms of zeolite of type A was investigated by various electrochemical methods. It is shown that the behaviour is typical of electrolytic systems but also with anodic and cathodic passivation of platinum electrodes. The electrode processes and the mechanism of charge transport through the electrolyte and the layer adjacent to the electrode are consisdered.

Kinetics of tantalum hydriding : the effect of palladization

Abstract The kinetics of tantalum hydriding under isothermal conditions, at six different temperatures in the range 573–823 K, was investigated. The kinetic parameters: rate constant, reaction order, activation energy and pre-exponential factor, were determined. The reaction order was found to amount 1, rate constants were found to vary in the range 5×10−6–2×10−4 s−1 in the temperature range under investigation, and an apparent activation energy was estimated to amount 56.2 kJ/mol Ta. No change in kinetical parameters was registered after multiple repetition of hydriding/dehydriding cycles. The same investigations were repeated with palladized samples. The palladization caused an abrupt increase in the rate constant value, but maximal hydriding capacity was found to remain unaffected.

Kinetic and thermodynamic research of hydrogen absorption by CoZr and NiZr alloys

Abstract The hydride formation by CoZr and NiZr alloys was investigated by differential scanning calorimetry method. Obtained thermograms were analysed and rate constants of hydrogen absorption, existing in stages 1–3, were determined. Activation energies of each of this stage, absorption enthalpy and frequency factor were also determined. It is shown that activation energies of the first stage are much higher than those of second and third one. The activation process of hydrogen is considered along with a new phase formation in already formed hydride in hydrogen atmosphere. X-ray analysis showed structural changes in NiZr alloys during activation.

Kinetics of thermal devitrification of some titanium amorphous alloys

A thermal investigation concerning the behaviour of amorphous alloys of 51% Cu, 10% Ni and 39wt % Ti (denoted by A) and of 13% Cu, 27% Ni and 60 wt % Ti (denoted by B) was performed using the differential scanning calorimetric method, alternatively in hydrogen and nitrogen atmosphere. It was shown that the crystallization (devitrification) is carried through in two and three different steps for alloy A and alloy B, respectively, within the temperature interval 573 to 823 K (300-550° C). Activation energies of each individual crystallization step, as well as frequency factors, rate constants, half-times and enthalpies of the crystallization reactions were determined.

Protonic conductivity of solid UO2HPO4·3H2O

Abstract The electrical and electrochemical properties of the solid ionic conductor UO 2 HPO 4 ·3H 2 O were investigated within the temperature range from room temperature up to 673 K. The conductivity of a nondehydrated sample within the temperature range from 303 K (6.5 × 10 -3 S/m) to 350 K (2.5 × 10 -2 S/m), with the activation energy of 23 kJ/mol, is a consequence of the presence of crystal water, and originates, mostly, from the fast movement of protons across the network of molecules of water in a tunneling mechanism pattern. When heated, the conductor loses crystal water, which leads to a final change in the nature of the conductor and in the conductivity mechanism. The conductivity of the dehydrated salt UO 2 HPO 4 (2.5 × 10 -4 S/m at 488 K), with the activation energy 44.04 kJ/mol, is considerably lower than the one mentioned above and can be attributed, to a great extent, to the movement of protons, most likely by a tunneling mechanism, through the less favourable structure formed by phosphate groups of the dehydrated salt. On being heated above 623 K for a certain time, the acid phosphate transforms into pyrophosphate, the conductivity of which is lower than that mentioned above. By means of electrochemical methods, the electrode processes were studied. It has been shown that H + and UO 2+ ions are reduced at the cathode, while the phosphate groups oxidize at the anode and O 2 is evolved.

Polarographic determination of arsenic in steel

A method for the polarographic determination of arsenic in steel is described. It consists of the reduction of arsenic(V) to arsenic(III) in 9·5 N hydrochloric acid solution by means of potassium iodide in the presence of ascorbic acid, and of the extraction of arsenic(III) with chloroform. Arsenic is then re-extracted from chloroform with 0·2 M ascorbic acid and determined polarographically with the same ascorbic acid solution as supporting electrolyte. The method was checked on NBS steel standards and synthetic mixtures. Arsenic contents greater than 0·01 per cent. can thus be determined easily without concentrating in the re-extraction step.