Hans Pink

Preparation of fast detecting SnO2 Gas sensors.

The oxidation of reactive gases at the surface of a semiconducting metal oxide is accompanied by a change in the electrical conductance. The effect was first used to detect gaseous components in air by Seiyama in 1964. In the meantime, numerous materials have been studied in attempts to optimize this effect, and at this time tin oxide seems to be a material with favorable properties. The very simple mounting of a detector on this base is a great advantage. But unfortunately some disadvantages still prevent the common application of this effect for the exact measurement of gases. One of them is the long time the detector needs to tend to the 90 % value and to regenerate. Using the method of spray pyrolysis, we prepared thin film SnO2 sensors which in contact with gas tend to the 90 % value within 30 ms and regenerate in less than 10 s.

Operation and Stability of SnO2 Gas Sensors

Solid state gas sensors based on thin films of SnO2 can be very sensitive to small amounts of hazardous gases like hydrocarbons and carbonmonoxide. There are two models to explain the observed change in conductance, when the sensors are exposed to the gases: (1) Adsorption of the gas molecule at the surface and electron transfer from the gas molecule to the SnO2 or (2) adsorption of the gas followed by a reaction of the gas with previously adsorbed oxygen. The conductivity change is caused by the released electron which was captured by the oxygen during its adsorption. Experiments were carried out which support the second model. In order to get a stable gas sensor it is necessary to keep the surface of the SnO2 thin film in such a condition that the oxygen adsorption/desorption process can operate for a long time.

The range of transferred hyperfine interactions in EuS

Abstract We report on a Mossbauer study of the transferred hyperfine fields at the 151 Eu nuclei in the Eu 1−x Sr x series of mixed compounds. The totally transferred hyperfine field amount −(5.3 ± 0.3) T corresponding to saturation values for the hyperfine field of −(33.4 ± 0.2) T for x = 0 and −(28.1 ± 0.2) T extrapolated for x = 1. Our results strongly support fairly long range magnetic interactions instead of the generally accepted model covering only nearest and next nearest neighbour interactions.

The spin-glass-to-ferromagnetism transition in the diluted Heisenberg system EuxSr1−xS

Abstract Neutron scattering and susceptibility measurements are used to study magnetic ordering in Eu x Sr 1−x S near the onset of ferromagnetism at x c = 0.51. The results provide evidence for a spin-glass state even above x c up to x ≈ 0.65 at low T.

Static magnetization and critical behaviour of EuxSr1−xS single crystals

Abstract The critical behaviour of the diluted Heisenberg ferromagnetic system Eu x Sr 1 − x S has been investigated with static magnetization measurements. The well-known effects associated with magnetic dilution are also confirmed for this system: 1. 1) increase f the effective critical exponents β, γ and δ, 2. 2) increase of the critical range, 3. 3) invariance of the Widom relation γ = β ( δ − 1). Most surprising is a linear decrease of the Schofield critical temperature b ∗ -1 = ( (δ − 1)(1 − 2β) (δ − 3) ) 1 2 with dilution. This quantity obeys, approximately, the simple empirical relation b ∗ -1 = T c (x) θ(x) .

The magnetic interaction potentials in ionic Gd3+ and Eu2+ compounds

Abstract We have measured the paramagnetic ordering temperature, θ, for a number of ionic Gd 3+ and Eu 2+ compounds in which the mean distance between the magnetic ions R varies between 0.35 and 0.75 nm. For both ions, the experimental dependence, θ( R ), resembles a chemical binding potential but with different signs (for small R , θ is negative for Gd 3+ , but positive for Eu 2+ ). Since Gd 3+ and Eu 2+ have the same electronic configuration (4f 7 5s 2 5p 6 ) and the same magnetic state ( 8 S 7 2 ), the sign of the magnetic interaction must be changed by the additional 5d electron which the Gd 3+ ion transfers to the ligands.

The influence of iron substitution on the properties of magnetic semiconducting thiospinels

Abstract In order to investigate the influence of iron substitution on the electrical and magnetic properties of semiconducting chromium spinels, the two pentatomic mixed systems (Cu + 0.5 In 3+ 0.5 ) 1- x Fe 2+ x Cr 2 S 4 , O ⩽ x ⩽, and Co 2+ 1- y (Cu + 0.5 Fe 3+ 0.5 ) y Cr 2 S 2 O ⩽ y ⩽ 1 were prepared. Single-phased spinels were obtained in the whole composition range. Curie temperature measurements show that in the former case T c -values are much below room temperature. In the latter case, T c increases about linearly with the iron content y and values around room temperature are observed. Conductivity measurements on hot-pressed pellets show all these samples to be semiconductors. Near T c negative-magnetoresistance effects are observed. This means that magnetic semiconductors with T c and the associated negative magnetoresistance effect around room temperature can be prepared by controlling the valence state of the inserted irons.