Konrad Colbow

Light scattering and turbidity measurements on lipid vesicles

The dynamic behaviour of model membranes in the form of sonicated liposomes in excess water was studied by means of 90 degrees C light scattering and turbidity measurements. Computer calculations based on the Rayleigh-Gans theory of light scattering were used to estimate the average size of lipid vesicles dispersed in water, taking into account the various structures of the vesicles. Normal reversible changes in the scattered light intensity and turbidity with temperature could be accounted for mainly by the changes in the refractive index of the lipid and irreversible anomalous changes were explained on the basis of fusion of smaller aggregated vesicles.

Algorithms to improve the selectivity of thermally-cycled tin oxide gas sensors

The conductance of a commercial tin oxide gas sensor (TGS#812) is measured as a function of concentration for a number of different gases under conditions of thermal cycling. This information is used to examine the criteria that could be used to improve the selectivity of detection. Different conductance-time curves (signatures) are seen for the various gases tested, which show gas-specific concentration dependences. Algorithms are presented that can distinguish the responses to propane, carbon monoxide or hydrogen from each other and from other gases (alcohols) over wide ranges of concentration. Irreversible poisoning effects occur under long-term exposure of the sensor to strong reducing gases, such as high concentrations of CO or H2. However, the conditions causing poisoning do not apply in most practical applications of gas sensors.

Signal-shape analysis of a thermally cycled tin-oxide gas sensor

Abstract The conductance versus time responses of a thermally cycled Figaro #813 gas sensor have been measured for clean laboratory air with the addition of several gases (hydrogen, propane, isopropanol and carbon monoxide) and their bicomponent mixtures. Output sensor signals are represented as linear combinations of two or more Gaussian functions. It is found that the positions of the maxima of these functions are characteristic features of a gas, providing a good criterion for the selective detection of inflammable or poisonous gases. The potential application of sensor signal deconvolution with Gaussian functions for quantitative analysis of two-component gas mixtures and its limitations are demonstrated.

General characteristics of thermally cycled tin oxide gas sensors

The response, measured as conductance, of thermally cycled tin oxide gas sensors was studied to determine the potential inherent in this technique with respect to increased selectivity and sensitivity. It was found that the range of cycle temperature as well as the period of the cycle had a critical effect on the amount of information that could be extracted from the conductance versus time curves. Significantly different conductance curve shapes were seen for such gases as propane, carbon monoxide and ethanol when an offset sinusoidal waveform was used to drive the sensor heater. The effect of varying the concentration of the reducing gas was examined both to check the effect on selectivity between gases and to determine the functional relationship between conductance and gas concentration as related to the time in the cycle period. It was found that the conductance change at the low-temperature section of the thermal cycle can vary at a rate faster than linear for low concentrations, and because the clear air conductance is very low in these sections the thermally cycled sensor is extremely sensitive to reducing gas.

Chromium oxide gas sensors for the detection of hydrogen, oxygen and nitrogen oxide

Abstract The gas sensitivity of chromium oxide, a semiconductor metal oxide, to hydrogen in air and to oxygen in hydrogen is presented and the operation principle is described. It is shown that when this oxide is doped Pd promoter, it has high sensitivity to hydrogen concentrations of a few ppm in air. When this oxide is doped with Pt and In it acquires room-temperature sensitivity. Sensors tested in flowing forming gas (50–100 cm 3 min −1 detect oxygen down to a few ppm concentration. It is shown that Pd-doped sensors have a switching characteristic at hydrogen/oxygen stoichiometric ratios of 1:1, resulting in a very sharp change in their conductance. It is found that the role of In, an n-type promoter, is to transform Cr 2 O 3 from a high-resistivity p-type to a low-resistivity n-type oxide. Pt provides room-temperature sensitivity due to its thin surface oxide layer. In flowing forming gas the sensors detect oxygen concentrations of 50 ppm at room temperature, but at about 100 °C the oxygen sensitivity increases and a concentration of a few ppm is detectable.

Spray pyrolysis elecrochromic WO3 films: Electrical and X-ray diffraction measurements

Abstract The carrier concentration and resistivity of electrochromic pyrolytic spray deposited tungsten trioxide films were measured. The resistivities of uncoloured films deposited at substrate temperatures between 200 and 400°C were found to be in the range 103–10−2 ω m and decreased by two to eight orders of magnitude after colouration. The colouration time was found to be dependent on thickness and deposition temperature of the oxide films. X-ray diffraction (XRD) studies of polycrystalline films prepared at temperatures higher than 350°C revealed structural changes during the electrochromic process. The XRD peaks for coloured films could be assigned to hydrogen tungsten bronze, and the structure of bleached films changed to the structure of the original uncoloured state.

Selective thermally cycled gas sensing using fast Fourier-transform techniques

Abstract The conductance versus time signatures of three types of thermally cycled gas sensors have been measured for a number of different reducing gases over a wide range of concentrations. A fast Fourier-transform data analysis technique is used to produce first-harmonic polar plots of phase angle versus magnitude of both single gases and mixtures as a function of concentration. It is found that this produces a good separation among many of the gases tested and leads to a clear selective criterion for the detection of single unknown gases. Gas mixtures can sometimes be distinguished as well, but with less reliability.

Theory of photocurrent in semiconductor‐electrolyte junction solar cells

Expressions are derived for the current‐voltage characteristics of semiconductor‐electrolyte junctions. Charge transfer kinetics, surface recombination, recombination in the quasineutral region and in the depletion region as well as the effect of the incident illumination on the minority carrier distribution in the semiconductor are included in the model. It is shown that surface transfer velocity for minority carriers is a very important parameter that determines the photocurrent of the cell. The dependence of the photoresponse on the light intensity is shown to be a diagnostic tool in determining the efficiency of charge transfer at the surface.

Analysis of temperature-jump chlorophyll fluorescence induction in plants

A newly observed general chlorophyll fluorescence induction effect in plants is described. Fluorescence yield can rise through as many as four different phases (alpha, beta, gamma, ) in the dark, when intact cells or leaves are rapidly heated (within approx. 2.5 s) from 20 to 40-50 degrees C. An analysis of this temperature-jump fluorescence induction in Scenedesmus obliquus leads to the following: 1. Phase alpha is due to removal of S-quenching and appears to be related to heat deactivation of the water-splitting enzyme system. With prolonged heating, irreversibility of alpha upon recooling reflects irreversible damage to the water-splitting enzyme system. 2. beta is independent of the S-states and of the redox state of primary System II acceptor Q. It is suggested that beta parallels functional separation of Q from the System II trapping centre. This effect is highly reversible. 3. gamma and beta reflect reduction of primary System II acceptor Q by a heat-induced endogenous reductant, which is probably identical to hydrogenase. Critical temperatures for pronounced alpha and beta phases differ markedly in different plants. Possible correlations between temperature-jump fluorescence inductio, thylakoid membrane lipid composition, lipid phase transition and lipid-protein interactions are discussed.

Surface silver clusters as oxidation catalysts on semiconductor gas sensors

Abstract We have investigated silver clusters on SnO2 thin-film surfaces by scanning electron microscopy (SEM), Auger electron spectroscopy (AES) and X-ray diffraction (XRD). These clusters act as catalytic activators on SnO2 gas sensors and improve sensor performance for H2 detection compared to untreated surfaces. The sensitivity is found to be dependent on the operating temperature of the sensors and the amount of Ag additives, which influences the cluster details. Cluster formation occurs reproducibly on heating the sample to about 350 °C, followed by cooling to 300 °C. The experimental results show that the best sensor performance is achieved by evaporating an Ag layer of 4 to 6 nm thickness and operating the sensor at 300 °C.