Georgios Dimitrakis

Electromagnetic simulations of microwave heating experiments using reaction vessels made out of silicon carbide

There is a growing body of literature which reports the use of silicon carbide vessels to shield reaction mixtures during microwave heating. In this paper we use electromagnetic simulations and microwave experiments to show that silicon carbide vessels do not exclude the electric field, and that dielectric heating of reaction mixtures will take place in addition to heat transfer from the silicon carbide. The contribution of dielectric heating and heat transfer depends on the dielectric properties of the mixture, and the temperature at which the reaction is carried out. Solvents which remain microwave absorbent at high temperatures, such as ionic liquids, will heat under the direct influence of the electric field from 30-250 degrees C. Solvents which are less microwave absorbent at higher temperatures will be heated by heat-transfer only at temperatures in excess of 150 degrees C. The results presented in this paper suggest that the influence of the electric field cannot be neglected when interpreting microwave assisted synthesis experiments in silicon carbide vessels.

A system for traceable measurement of the microwave complex permittivity of liquids at high pressures and temperatures

A system has been developed for direct traceable dielectric measurements on liquids at high pressures and temperatures. The system consists of a coaxial reflectometric sensor terminated by a metallic cylindrical cell to contain the liquid. It has been designed for measurements on supercritical liquids, but as a first step measurements on dielectric reference liquids were performed. This paper reports on a full evaluation of the system up to 2.5 GHz using methanol, ethanol and n-propanol at pressures up to 9 MPa and temperatures up to 273 °C. A comprehensive approach to the evaluation of uncertainties using Monte Carlo modelling is used.

Continuous direct on-line reaction monitoring of a controlled polymerisationvia dielectric measurement

This paper reports the application of an extremely facile methodology for monitoring the progress of chemical reactions. This has been exemplified by successfully following the progress of a controlled ring opening polymerisation of e-caprolactone which has been successfully monitored at high temperature (>100 °C), using a direct, on-line measurement of the system dielectric properties. The data was obtained via the use of a coaxial probe in a standard quick fit reaction flask. The on-line measurement has been related to a calibration curve and from this comparison it has been shown that dielectric data can be used to predict the molecular weight of the polymer at a particular point in the reaction. This in turn allowed isolation of a product with desired molecular weight and polydispersity index values, by enabling the reaction to be terminated prior to the point where reaction control is lost due to the appearance of side reactions. These on-line measurements were corroborated by comparison to conventional and accepted off-line measurement techniques.

Probing the microwave interaction mechanisms and reaction pathways in the energy-efficient, ultra-rapid synthesis of tungsten carbide

Tungsten carbide, WC, can be prepared by microwave (MW) synthesis in the solid state from its component elements in air on timescales that are orders of magnitude faster than those achieved by conventional heating methods. In a multimode cavity (MMC) MW reactor, the synthesis can be performed in less than 30 min; in a single mode cavity (SMC) reactor the reaction time is reduced to <1 min. Combinations of optical pyrometry, powder X-ray and neutron diffraction, thermal imaging and dielectric measurements demonstrate how these syntheses are self-terminating, initiated at elevated temperature and proceed via the intermediate formation of the sub-carbide W2C. The result is phase-pure, dense, crystalline and stoichiometric WC (hexagonal Pm2, a = 2.90567(3) A, c = 2.83641(3) A, Z = 1).

Continuous and direct ‘in situ’ reaction monitoring of chemical reactions via dielectric property measurement: controlled polymerisation

This paper demonstrates that direct, “in situ” measurement of a reaction mixture using a coaxial probe technique can be used to accurately follow the progress of a chemical reaction. Such a system was shown to clearly indicate the presence/onset of and define the magnitude of key reaction parameters such as induction periods and end-points over a broad range of temperatures and viscosities. Thus it allowed the reaction to be conducted for the ideal time period, so maximising reactor through-put, energy efficiency and end product quality. Furthermore, by relating these ‘in situ’ measurements to a pre-prepared calibration curve, key experimentally achieved reaction rates could be determined. Finally, these continuously acquired, non-intrusive ‘in situ’ measurements were validated by comparison to conventional and industry accepted off-line measurement techniques.

Enhanced ‘in situ’ catalysis via microwave selective heating: catalytic chain transfer polymerisation

An extremely facile, single stage, ‘in situ’, Catalytic Chain Transfer Polymerisation (CCTP) process has been identified, where the optimal polymerisation process was shown to depend upon a combination of catalyst characteristics (i.e. solubility, sensitivity, activity) and the method of heating applied. In comparison to the current benchmark catalyst, the preparation of which is only about 40% efficient, this represents a significant increase in waste prevention/atom efficiency and removes the need for organic solvent. It was also shown possible to significantly reduce the overall ‘in situ’ reaction cycle time by adopting different processing strategies in order to minimise energy use. The application of microwave heating was demonstrated to overcome system diffusion/dilution issues and result in rapid, ‘in situ’ catalyst formation. This allowed processing times to be minimised by enabling a critical concentration of the species susceptible to microwave selective heating to dominate the heat and mass transfer involved.

Facile Determination of Molecular Structure Trends in Amphiphilic Core Corona Star Polymer Synthesis via Dielectric Property Measurement

The use of dielectric property measurements to define specific trends in the molecular structures of poly(caprolactone) containing star polymers and/or the interbatch repeatability of the synthetic procedures used to generate them is demonstrated. The magnitude of the dielectric property value is shown to accurately reflect: (a) the number of functional groups within a series of materials with similar molecular size when no additional intermolecular order is present in the medium, (b) the polymer molecular size for a series of materials containing a fixed core material and so functional group number, and/or (c) the batch to batch repeatability of the synthesis method. The dielectric measurements are validated by comparison to spectroscopic/chromatographic data.

Molecular Differentiated Initiator Reactivity in the Synthesis of Poly(caprolactone)-Based Hydrophobic Homopolymer and Amphiphilic Core Corona Star Polymers.

Macromolecules that possess three-dimensional, branched molecular structures are of great interest because they exhibit significantly differentiated application performance compared to conventional linear (straight chain) polymers. This paper reports the synthesis of 3- and 4-arm star branched polymers via ring opening polymerisation (ROP) utilising multi-functional hydroxyl initiators and Sn(Oct)2 as precatalyst. The structures produced include mono-functional hydrophobic and multi-functional amphiphilic core corona stars. The characteristics of the synthetic process were shown to be principally dependent upon the physical/dielectric properties of the initiators used. ROP’s using initiators that were more available to become directly involved with the Sn(Oct)2 in the “in-situ” formation of the true catalytic species were observed to require shorter reaction times. Use of microwave heating (MWH) in homopolymer star synthesis reduced reaction times compared to conventional heating (CH) equivalents, this was attributed to an increased rate of “in-situ” catalyst formation. However, in amphiphilic core corona star formation, the MWH polymerisations exhibited slower propagation rates than CH equivalents. This was attributed to macro-structuring within the reaction medium, which reduced the potential for reaction. It was concluded that CH experiments were less affected by this macro-structuring because it was disrupted by the thermal currents/gradients caused by the conductive/convective heating mechanisms. These gradients are much reduced/absent with MWH because it selectively heats specific species simultaneously throughout the entire volume of the reaction medium. These partitioning problems were overcome by introducing additional quantities of the species that had been determined to selectively heat.

Wet steam measurement techniques

Abstract In recent years a greater need for power station efficiency has become evident; improving turbine blade efficiency is one of the methods proposed. This efficiency depends upon the wetness of the steam that comes into contact with the blades of the low-pressure turbine stage in general and all turbines in nuclear power generation. Therefore, measurement of the moisture content of the steam in real time in conjunction with an accurate measure of steam velocity can give an overall mass flow re-entering the turbine, allowing for a feedback control. The system can rely on one technique that can measure suspended droplets and wall-bound liquid film, or a combination of techniques can operate together. This work gives a comprehensive review of the different techniques used to measure the moisture content including the liquid film and moisture content and techniques that can give measurements on both simultaneously. Each technique has its strengths and weaknesses, and they were analysed to see which technique works best overall and which techniques can be used together.

Traceable measurement and imaging of the complex permittivity of a multiphase mineral specimen at micron scales using a microwave microscope

This paper describes traceable measurements of the dielectric permittivity and loss tangent of a multiphase material (particulate rock set in epoxy) at micron scales using a resonant Near-Field Scanning Microwave Microscope (NSMM) at 1.2GHz. Calibration and extraction of the permittivity and loss tangent is via an image charge analysis which has been modified by the use of the complex frequency to make it applicable for high loss materials. The results presented are obtained using a spherical probe tip, 0.1mm in diameter, and also a conical probe tip with a rounded end 0.01mm in diameter, which allows imaging with higher resolution (≈10µm). The microscope is calibrated using approach-curve data over a restricted range of gaps (typically between 1% and 10% of tip diameter) as this is found to give the best measurement accuracy. For both tips the uncertainty of scanned measurements of permittivity is estimated to be±10% (at coverage factor k=2) for permittivity ⪝10. Loss tangent can be resolved to approximately 0.001. Subject to this limit, the uncertainty of loss tangent measurements is estimated to be±20% (at k=2). The reported measurements inform studies of how microwave energy interacts with multiphase materials containing microwave absorbent phases.