Julian J. Murphy

Sol-gel materials for gas-sensing applications

Abstract High temperature sintering of porous pellets of metal oxides to form flammable gas sensors remains one of the most striaghtforward and popular fabrication methods in use today, but is unsatisfactory in may wasys including lack of control over structure and morphology of the pellets. In addition, the chemical and physical role of catalytic dopants is often unclear and further sintering during operation can lead to long-term drift and unreliable behaviour. We have been exploring the use of alternative fabrication techniques, for the development of highly sensitive flammable gas detectors, based on the sol-gel process. The main advantage of this technique over conventional processing technologies is the use of homogenous, multi-component systems which can be prepared to a high degree of purity by mixing the molecular precursor solutions, and the reduction in fabrication temperature leading to unusual galsses or ceramics with better-defined properties. Several advantages in the development of highly sensitive flammable gas sensors are to be expected. Deliberate addition of impurities can be carefully controlled and low temperature fabrication should allow greater control over the structure, stoichiometry and morphology of the sensors. Both of these factors will contribute towards low background conductivity and high purity for high sensitivity. Finally the high porosity and large surface of glassy materials produced by sol-gel methods should enhance sensitivity in mechanims dominated by surface phenomena. In this paper we present details of the preparation of novel tin oxide thin-film sensors and initial results of response to a variety of organic solvents and common flammable gases. A comparison with conventional, commercially available tin dioxide (Taguchi) flammable gas sensors will be given.

The stability of polysiloxanes incorporating nano-scale physical property modifiers

Abstract Reported here is the synthesis and subsequent characterization of the physical and chemical properties of novel polysiloxane elastomers modified with a series of polyhedraloligomericsilsequioxane (POSS) molecular silicas. The physical properties of the formulated nanocomposite systems have been characterized with a combination of dynamic mechanical analysis (DMA), broadband dielectric spectroscopy (BDS) and confocal Raman microscopy. The results of the physical property characterization demonstrate that the incorporation of low levels (1–4% by wt.) of POSS particles into the polysiloxane network leads to significant improvements in the mechanical properties of the elastomer and significantly alters the motional chain dynamics of the system as a whole. The results of studies performed to assess the long-term stability of these novel nanocomposite systems have demonstrated that POSS physical property modifiers can significantly alter the thermal stability of polysiloxane elastomers. Physically dispersed POSS has also been shown in some cases to be both mobile and disruptive within the polysiloxane networks, agglomerating into domains on a micron scale and migrating to the surface of the elastomers. This work demonstrates both the potential of POSS nanoparticles as physical property modifiers and describes the effects of POSS on the physical and chemical stability of polysiloxane systems.

Synthesis and characterisation of pyrene-labelled polydimethylsiloxane networks: towards the in situ detection of strain in silicone elastomers

Pyrene-substituted polyhydromethylsiloxanes (PHMS-Pyx) were synthesised by the hydrosilylation reaction of prop-3-enyloxymethylpyrene with polyhydromethylsiloxane (Mn = 3700). The ratio of pyrene substituent to Si–H unit was varied to afford a range of pyrene-functionalised polysiloxanes. These copolymers were subsequently incorporated into polydimethylsiloxane (PDMS) elastomers by curingvia either Pt(0) catalysed hydrosilylation with divinyl-terminated PDMS (Mn = 186) and tetrakis(dimethylsiloxy)silane, or Sn(II) catalysed condensation with α,ω-dihydroxyPDMS (Mn = 26 000) and tetraethoxysilane. An alternative method involving the synthesis and integration of [3-(pyren-1-ylmethoxy)propyl]triethoxysilane (Py-TEOS) into PDMS elastomers was also investigated: a mixture of α,ω-dihydroxyPDMS (Mn = 26 000), tetraethoxysilane, and Py-TEOS was cured using an Sn(II) catalyst. Certain of the resulting fluorescent pyrene-labelled elastomers were studied by differential scanning calorimetry and dynamic mechanical analysis. No significant changes were observed in the thermal or mechanical properties of the elastomers containing pyrene when compared to otherwise identical samples not containing pyrene. All of the pyrene-containing elastomers were demonstrated to be fluorescent under suitable excitation in a photoluminescent spectrometer. Two of the elastomers were placed in a photoluminescence spectrometer and subjected to cycles of extension and relaxation (strain = 0–16.7%) while changes in the emission spectra were monitored. The resulting spectra of the elastomer containing the PHMS-Py50 copolymers were variable and inconsistent. However, the emission peaks of elastomers containing Py-TEOS displayed clear and reproducible changes in fluorescence intensity upon stretching and relaxation. The intensity of the monomer and excimer emission peaks was observed to increase with elongation of the sample and decrease upon relaxation. Furthermore, the ratio of the intensities of the excimer : monomer peak decreased with elongation and increased with relaxation. In neither case was there appreciable hysteresis, suggesting that fluorescent labelling of elastomers is a valid approach for the non-invasive in situ monitoring of stress and strain in such materials.

Volatile evolution from room temperature cured polysiloxane rubber induced by irradiation with He2+ ions

Abstract A room temperature cured polysiloxane rubber (Rhodorsil RTV5370) has been irradiated using an accelerated beam of He 2+ ions. Such an irradiation simulates the effects of large alpha radiation doses. A mass spectrometer linked directly to the sample chamber allowed the analysis of volatile species evolved as a result of exposure. The polydimethylsiloxane rubber showed high resistance to radiation damage when exposed to He 2+ ions at doses up to 3.5 MGy. At higher doses, the cyclic hexamethylcyclotrisiloxane was evolved and is indicative of damage to the main chain. The cyclic octamethylcyclotetrasiloxane was only observed at very high doses (10 MGy and above) and is indicative of significant head-to-tail unzipping reactions. Methane, benzene and carbon dioxide were the three main gases evolved. The ratio of phenyl (2%) to methyl groups (93%) within RTV5370 rubber is very small but the amount of benzene evolved was found to be significant. The results suggest that the aromatic groups within the rubber appear to have a much greater susceptibility to radiation induced volatile evolution than other groups making up the polymer structure. In addition, the depletion of phenyl groups (from the reduction in evolution of benzene) from the irradiated zone corresponds to an increase in damage of the siloxane linkages (from the increased evolution of octamethylcyclotetrasiloxane) suggesting the aromatic groups offer a protective action from the incident radiation.

Simple model of the lithographic response of “Hatzakis” chemically-amplified resists

A simple mathematical model has been employed to analyze quantitatively the crosslinking process in chemically amplified negative resists based on the combination of an epoxidized novolac resin and a photoacid generator. Experimental evaluation of a number of resists employing different resins has shown that sensitivity and contrast increase concomitantly. A resist formulation based on an experimental Dow resin designated DQ9 has demonstrated exceptional contrast and a sensitivity well matched to many e-beam systems.

Electron beam resists based on oxirane functionalised polystyrenes

Copolymers consisting of styrene and 4-ethenyl-phenyloxirane (epoxystyrene) can be synthesised by radical copolymerisation. The flexibility offered by this simple method of synthesis allows the production of a variety of copolymers. The possibility of altering the lithographic parameters by modification of the copolymer makes this method of production highly attractive for the purposes of lithographic optimisation. More so than for the analogous systems based upon epoxy novolak resins which suffer because a range of different polymers that, would allow a worthwhile structure/process optimisation, is not available.

Evaluation of halomethylated poly(methylphenylsilane)s as electron-beam resists

Polysilane analogues of halomethylated poly(styrene)s, chloromethylated and bromomethylated poly(methylphenylsilane), have been prepared from the parent polymer by reaction with the appropriate halomethyl methyl ether. The polymers undergo a single-stage crosslinking reaction when irradiated with 20 kV electrons. As electron beam resists they operate in negative-working mode but their performance is poor in comparison to the corresponding poly(styrene) derivatives. The low lithographic sensitivities and attainable contrasts are shown to arise as a consequence of a competitive chain scission reaction which in the case of the bromomethylated system increases with increasing bromomethyl content. The radiation chemistries of the systems are rationalised in terms of modifications of the crosslinking and scission mechanisms that are thought to operate in the corresponding resists based on poly(chloromethylstyrene-stat-styrene).

Radiation chemistry and the lithographic performance of chemical amplification resists formulated from poly(4-epoxystyrene-stat-styrene) and a photoacid generator

Copolymers of styrene and 4-epoxystyrene in formulations with triphenylsulfonium hexafluoroantimonate as a photoacid generator undergo a crosslinking reaction by a chain mechanism when irradiated with 20 keV electrons and hence act as negative-working electron-beam resists. The copolymers have been prepared by a free radical mechanism over the entire composition range and the lithographic performance of the materials has been evaluated. The sensitivities of the resist formulations are shown to correlate with the epoxystyrene content of the copolymers in accordance with a simple model of the radiation chemistry of the system. Contrast variations are explained in terms of the statistical structure of the copolymer and it is demonstrated that at low epoxystyrene contents the systems behave in accordance with an unsensitised single-stage crosslinking mechanism. The copolymers with epoxystyrene contents greater thanca. 8% have such high lithographic sensitivities and show such a good tolerance of processing variations as to commend the optimisation of their performance with a view to their subsequent application as electron-beam resists.

Volatile Evolution from Polymer Materials Induced by Irradiation with Accelerated He ++ Ions

Experimentally investigating ageing caused by irradiation with energetic particles is very difficult. Radioactive sources can be employed but these are difficult to handle and contaminate the material being irradiated precluding subsequent chemical and physical characterisation. The penetration of energetic particles also tends to be small so any change is localised in the near surface region so only a small amount of material is irradiated. Analysing changes in such thin layers causes a number of problems. To simulate ageing induced by particle radiation polymer samples have been exposed to fast He ++ ions in an accelerated ion beam. The ions pass through a 10μm thick window of Havar foil before impacting upon the sample. Volatile species evolved from the materials upon bombardment are contained within the irradiation chamber by the foil window. Analysis of such species is shown to be a highly sensitive probe for investigating chemical changes in the exposed materials. A number of important chemical changes induced in polymer materials have been identified. Trends in the relative rates of volatile evolution have been identified which correlate with chemical changes identified in other radiation experiments. As these experiments are performed at far slower irradiation rates the large acceleration factors used in ion beam irradiation are discussed along with the implications for using ion beams to simulate alpha particle irradiation.

Multi-factor authentication using accelerometers for the Internet-of-Things

Embedded and mobile devices forming part of the Internet-of-Things (IoT) need new authentication technologies and techniques. This requirement is due to the increase in effort and time attackers will use to compromise a device, often remote, based on the possibility of a significant monetary return. This paper proposes exploiting a devices accelerometers in-built functionality to implement multi-factor authentication. An experimental embedded system designed to emulate a typical mobile device is used to implement the ideas and investigated as proof-of-concept.