Michael L. Hitchman

Some recent trends in the preparation of thin layers by low pressure chemical vapour deposition

Abstract Recent trends in low pressure chemical vapour deposition (LPCVD) include the use of methods for initiating the deposition process other than thermal irradiation. These trends are reviewed and the use of ultrasonics is suggested as another technique with a number of practical advantages and with possibility of initiating novel reactions. Other trends involve applications for the deposition of a wider range of materials. In situ doping of polysilicon is an example, and the effect of deposition conditions on the characteristics of such layers is discussed and analysed.

Direct potentiometric monitoring of proteins

Abstract A simple, reliable, rapid and inexpensive method of monitoring proteins is based on direct potentiometry with metal electrodes which are electrochemically treated before each measurement. The principles behind the treatment processes are discussed and the results are illustrated with an application to protein chomatography.

Potentiometric monitoring of proteins: Part 2. Results with Electrochemical Cleaning of a Silver Electrode

Abstract Results are presented for the direct potentiometric determination of L -cysteine and several proteins with silver electrodes which are electrochemically treated before measurements. Comparisons are made with untreated and chemically cleaned electrodes in terms of Nernstian behaviour and equilibration times. The improved behaviour of the electrochemically treated electrodes is discussed in terms of a theoretical model.

Observations relating to oxygen permeability measurements on membranes

Abstract A cell for electrochemically measuring dissolved oxygen permeability coefficients ( P d ) has been constructed and tested successfully with Teflon ® FEP membranes of previously established P d values. Mixtures of n-butyl acrylate, N -vinyl-2-pyrrolidone and a crosslinking agent have been copolymerized to complete conversion via γ-irradiation. Thin discs of the products have been swollen in water to yield new hydrogels with equilibrium water contents (EWC) of 39–73%. The observed exponential increase in P d with EWC was of a similar form to that reported by other workers for chemically different hydrogels. At a fixed EWC, there was an increase in P d with thickness of membrane, in accord with the theoretical model for multilayer diffusive transport. However, the results also indicate the effect of the aqueous boundary layer on the measured value of P d . The need is demonstrated for measurements on highly permeable hydrogels with aqueous environments to be conducted under well-defined transport controlled conditions, if reliable values of P d are to be obtained.

A consideration of the effect of reactor parameters on the characteristics of layers prepared by low pressure chemical vapour deposition

Abstract Low pressure chemical vapour deposition (LPCVD) has significant performance advantages for the preparation of thin films as compared with chemical capour deposition in atmospheric pressure reactors. In particular, very uniform conformal films can be grown on closely spaced substrates. In the semiconductor industry this has resulted in a changeover to low pressure, hot wall systems for the growth of polycrystalline and amorphous layers. In this paper we illustrate the ways in which growth conditions in the reactor, and particularly, the chemistry of the CVD process can affect the growth rate, uniformity, composition, structure and properties of such layers.

Indirect potentiometric monitoring of proteins with a copper electrode

Abstract Preliminary results are given for the monitoring of proteins by indirect potentiometry with the Cu2+/Cu couple in a flow system. Electrochemical cleaning of the electrode, before measurement of the potential in the presence of proteins, allows reproducible determinations of proteins at the micromolar level. The effects of pH and chloride on the measured potentials are investigated.

Potentiometric monitoring of proteins: Part 1. Introduction and theory for a silver electrode

Abstract A brief survey of methods for monitoring of proteins is given, and the relative advantages and disadvantages of potentiometry are considered. A theoretical model is presented for direct potentiometric monitoring of a thiol-containing protein with a silver electrode. The model is used to indicate ways of improving the reproducibility and response of the electrode.

The determination of a carbamate insecticide in soil samples by differential pulse polarography

Abstract Differential pulse polarography is shown to be a simple and potentially useful method for monitoring the degradation of a carbamate insecticide (Bendiocarb; 2,3-isopropyli-denedioxyphenyl-N-methylcarbamate) in soil samples. Only extraction from the soil sample with dichloromethane and evaporation of the extract is needed prior to polarography. Calibration plots were linear over the range 10–50 mg l−1 at −0.94 V vs. Ag/AgCl in an acetate buffer of pH 5.0. There is no apparent interference from hydrolysis products or soil components.

A study of the effect of some pollutant gases on cathodes used in membrane-covered amperometric oxygen detectors

Abstract The effects of various gaseous pollutants on the electrochemical activity of materials used as cathodes for membrane-covered amperometric oxygen detectors are described. It is shown that gold and platinum cathodes are not unduly affected by concentrations of sulphur dioxide and chlorine many times greater than those likely to be encountered in test solutions, and that a gold cathode is also unaffected by hydrogen sulphide. A platinum cathode is rapidly and significantly poisoned on contact with hydrogen sulphide; an analysis of the fall in the rate of oxygen reduction as a function of time indicates that the poisoning occurs by the blocking of surface sites by sulphide. The effect of hydrogen sulphide on silver, nickel and nickel sulphide electrodes is also reported. Of these materials, only nickel sulphide is an effective electrocatalyst for oxygen reduction in the presence of sulphide.

A simple and inexpensive wash unit for low pressure CVD reactor tubes

Abstract Design considerations for a wash unit for low pressure CVD reactor tubes are discussed, and the construction of a simple, inexpensive unit is described. The cleaning of a polysilicon deposit off a furnace tube is used to illustrate the use of the unit.