Paul J. Brewer

The physical and chemical properties of electroless nickel–phosphorus alloys and low reflectance nickel–phosphorus black surfaces

Novel insights into the manufacture of nickel–phosphorus black surfaces by chemical etching of electroless-deposited Ni–P alloy has been achieved by examining the influence of pre-etch phosphorus composition and etching method on the resulting morphology, composition and reflectance of the black surface produced. An optimum phosphorus composition and etching regime to produce low reflectance blacks of 0.4% or lower in the visible region is proposed. Cross-sectional analysis of the etched surface has allowed, for the first time, an accurate determination of the scale of the enhanced morphologies produced and the thickness of the oxidised black layer itself. AFM studies have also provided information on the phase structure of the as-deposited Ni–P alloy.

Microfabricated planar glass gas chromatography with photoionization detection

We report the development of a microfabricated gas chromatography system suitable for the separation of volatile organic compounds (VOCs) and compatible with use as a portable measurement device. Hydrofluoric acid etching of 95x95mm Schott B270 wafers has been used to give symmetrical hemi-spherical channels within a glass substrate. Two matching glass plates were subsequently cold bonded with the channels aligned; the flatness of the glass surfaces resulted in strong bonding through van der Waals forces. The device comprised gas fluidic interconnections, injection zone and 7.5 and 1.4m long, 320microm internal diameter capillaries. Optical microscopy confirmed the capillaries to have fully circular channel profiles. Direct column heating and cooling could be achieved using a combination of resistive heaters and Peltier devices. The low thermal conductivity of glass allowed for multiple uniform temperature zones to be achieved within a single glass chip. Temperature control over the range 10-200 degrees C was achieved with peak power demand of approximately 25W. The 7.5m capillary column was static coated with a 2microm film of non-polar dimethylpolysiloxane stationary phase. A standard FID and a modified lightweight 100mW photoionization detector (PID) were coupled to the column and performance tested with gas mixtures of monoaromatic and monoterpene species at the parts per million concentration level. The low power GC-PID device showed good performance for a small set of VOCs and sub ng detection sensitivity to monoaromatics.

Surface modification of PDMS via self-organization of vinyl-terminated small molecules

Polydimethylsiloxane (PDMS) elastomers are widely used in soft lithography, microfluidics and biomedical applications as they combine a range of desirable chemical and physical properties. We studied the surface of PDMS modified using surface enrichment with long-chain alkenes. These alkenes were immobilized during crosslinking of the PDMS pre-polymer mixture cast against a template of matching surface energy. A range of different functional groups was introduced, including perfluorinated and oligoethylene glycol groups, alkyl chains and initiators for controlled radical polymerization. A detailed analysis of the composition of the modified PDMS surfaces was carried out using contact angle measurements, AFM, SIMS and XPS. The results demonstrate that the PDMS was enriched with small molecules near the surface and show that the functional molecules follow the template surface energy on patterned surfaces with edge resolutions equal to the template. By introducing hydrophilic alkenes receding contact angles on PDMS can be lowered below 5°, and we investigated how these surfaces rearrange in air due to the chain mobility of the PDMS backbone and PDMS short-chain fragments.

Accurate measurements of water vapor transmission through high-performance barrier layers

We report a new approach to measuring very low rates of water vapor transmission through high-performance barrier layers, based on detection of the water vapor by cavity ring-down infrared spectroscopy. It provides accurate and traceable measurements with a detection limit for water vapor transmission significantly below 1 × 10(-4) g/m(2)/day. The system is underpinned by dynamic reference standards of water vapor generated between 5 and 2000 nmol∕mol with an estimated relative expanded uncertainty of ±2%. It has been compared with other methods and demonstrates good comparability.

High-accuracy stable gas flow dilution using an internally calibrated network of critical flow orifices

A network of critical flow orifices has been developed to form a gas flow dilutor capable of stable and repeatable operation, which is not influenced by environmental conditions. When used with a novel self-calibration method it achieves dilutions of up to 31:1 with a relative standard uncertainty of ±0.1%. This new approach avoids the uncertainty generated by setting and controlling the flow which is present in devices based on variable flow elements.

Effect of Structural Design of Silver/Silver Chloride Electrodes on Stability and Response Time and the Implications for Improved Accuracy in pH Measurement.

The response time of thermal electrolytic Ag/AgCl reference electrodes is defined by the porous structure that limits the rate at which traces of any previous solution are diluted by any new solution environment. The electrode stabilisation time when transferred from one electrolyte to another has been shown to change when different structural designs to the conventional sphere of Ag/AgCl are used. Electrodes fabricated with cylindrical architectures of Ag/AgCl have shown improved stability and reach equilibrium faster than spherical electrodes. This work has positive implications for the accuracy and throughput of primary pH measurements made using the Harned cell.

Sensitivities of a standard test method for the determination of the pHe of bioethanol and suggestions for improvement.

An assessment of the sensitivities of the critical parameters in the ASTM D6423 documentary standard method for the measurement of pHe in (bio)ethanol has been undertaken. Repeatability of measurements made using the same glass electrode and reproducibility between different glass electrodes have been identified as the main contributors to the uncertainty of the values produced. Strategies to reduce the uncertainty of the measurement have been identified and tested. Both increasing the time after which the pHe measurement is made following immersion in the sample, and rinsing the glass electrode with ethanol prior to immersion in the sample, have been shown to be effective in reducing the uncertainty of the numerical value produced. However, it is acknowledged that the values produced using these modified approaches may not be directly compared with those obtained using the documentary ASTM method since pHe is defined operationally by the process used to measure it.

Preparation and Validation of Fully Synthetic Standard Gas Mixtures with Atmospheric Isotopic Composition for Global CO2 and CH4 Monitoring

We report the preparation and validation of the first fully synthetic gaseous reference standards of CO2 and CH4 in a whole air matrix with an isotopic distribution matching that is in the ambient atmosphere. The mixtures are accurately representative of the ambient atmosphere and were prepared gravimetrically. The isotopic distribution of the CO2 was matched to the abundance in the ambient atmosphere by blending (12)C-enriched CO2 with (13)C-enriched CO2 in order to avoid measurement biases introduced by measurement instrumentation detecting only certain isotopologues. The reference standards developed here have been compared with standards developed by the National Institute of Standards and Technology and standards from the WMO scale. They demonstrate excellent comparability.

Effect of Silver Annealing Conditions on the Performance of Electrolytic Silver/Silver Chloride Electrodes used in Harned Cell Measurements of pH

We have studied the long and short term stability of electrolytic Ag/AgCl electrodes fabricated from Ag wire that has been subjected to a range of different annealing conditions. At elevated temperatures, the presence of oxygen during the annealing process has been shown to be detrimental to the performance of electrodes produced. This phenomenon has been attributed to the dissolution of oxygen in the Ag lattice leading to structural changes in the Ag/AgCl electrode material. Electrodes prepared from Ag wire annealed in the absence of oxygen have shown no appreciable change in performance throughout the temperature range employed. This work has resulted in an improved understanding of the optimum annealing conditions required for Ag used in the preparation of electrolytic Ag/AgCl reference electrodes. This work has positive implications for the accuracy of Harned cell measurements of pH.

Sensitivities of Key Parameters in the Preparation of Silver/Silver Chloride Electrodes Used in Harned Cell Measurements of pH

A questionnaire was completed by fourteen world leading national metrology institutes to study the influence of several variables in the preparation of Ag/AgCl electrodes on the accuracy of Harned cell measurements of pH. The performance of each institute in the last decade has been assessed based on their results in eight key comparisons, organized by the Bureau International des Poids et Measures Consultative Committee for Amount of Substance, involving the measurement of pH of phosphate, phthalate, carbonate, borate and tetroxalate buffer solutions. The performance of each laboratory has been correlated to the results of the questionnaire to determine the critical parameters in the preparation of Ag/AgCl electrodes and their sensitivities with respect to the accuracy of pH measurement. This study reveals that the parameters most closely correlated to performance in comparisons are area of electrode wire exposed to the electrolyte, diameter and porosity of the Ag sphere prior to anodisation, amount of Ag converted to AgCl during anodisation, stability times employed for electrodes to reach equilibrium in solution prior to measurement, electrode rejection criteria employed and purity of reagents.