Microwave-matter effects in metal(oxide)-mediated chemistry and in drying

Microwave irradiation is a well-accepted heating technique for lab-scale organic synthesis but its application for large-scale operation is still limited. To determine the potential of microwave heating in producing fine chemicals beyond the kg-scale, the added value of this heating technique, compared to conventional heating, has been evaluated at accurately controlled conditions on lab-scale. The research described in this thesis focuses on comparing microwave heating with conventional heating for a series of heterogeneous reactions and for purification, i.e. drying. This enabled to elucidate factors determining the benefits of microwave-mediated technology. In Chapter 1 the state of the art in the application of microwave technology is discussed and the outline of the thesis is presented. In Chapter 2 the Grignard reagent formation, involving a heterogeneous metal, i.e. magnesium, is discussed. Microwave irradiation of magnesium turnings led to electrical discharges, which modify the surface and, therefore, the reactivity of magnesium. The influence of modifying the magnesium surface on the reactivity of the metal in the Grignard reagent synthesis was determined for a series of halo-compounds. The initiation time significantly shortened upon irradiating the reaction mixtures of relatively reactive (2-bromothiophene, 2-bromopyridine, bromobenzene, iodobenzene and n-octyl bromide) and moderately reactive (2-chlorothiophene and 2-chloropyridine) halo-substrates. In contrast, irradiating the reaction mixtures of non-reactive halogenated compounds (3-bromopyridine and n-octyl chloride) led to major magnesium carbide formation causing a reduced reactivity of the metal and prolonged initiation times. In Chapter 3 the influence of microwave heating on another heterogeneous organometallic reaction, the Reformatsky reaction, involving metallic zinc, is discussed. In this system microwave-induced electrical discharges caused major zinc carbide formation, irrespective of the presence of a species reactive towards zinc. The zinc carbide formation coated the zinc surface, which was responsible for inhibition of zinc insertion in acetate, propionate and isobutyrate esters. This zinc carbide formation limited the beneficial use of microwave heating in the Reformatsky reaction to such an extent that conventional heating has to be preferred. Information on the influence of microwave energy on a heterogeneous organometallic reaction involving metallic copper, the Ullmann coupling of 2-chloro-3-nitropyridine, is given in Chapter 4. In this case, microwaves did not seem to interact with the copper directly, limiting the impact of this heating mode. The reaction was optimized in terms of temperature, copper source, stoichiometry and solvent. Surprisingly, fine copper powder (45 µm) is a better metal source than traditional copper-bronze for this Ullmann carbon-carbon coupling. A ratio of 1:1 of copper to 2-chloro-3-nitropyridine resulted in reaction profiles similar to those resulting from an excess of copper. Switching solvent from N,N-dimethylformamide (DMF) to N,N-dimethylacetamide (DMA) or N-methyl-2-pyrrolidone (NMP) diminished reaction rates, prolonged initiation times, lowered yields and gave rise to the formation of 2,2'-oxybis(3-nitropyridine) as byproduct. Therefore, DMF is the preferred solvent for this Ullmann coupling. Comparison of microwave heating with conventional heating for the reactions performed at optimized conditions (in DMF at 110 °C), as well as under less ideal conditions (in DMA and NMP at various temperatures) revealed identical time-conversion histories, yields and selectivities. The results with magnesium, zinc and copper reveal that, although, the Grignard reagent formation, the Reformatsky reaction and the Ullmann coupling are very similar processes, the influence of microwave irradiation on the outcome of the process is not. In Chapter 5 the influence of microwave irradiation on a freshly prepared zirconium-based heterogeneous catalyst for the amide formation from a nitrile and an amine is presented. The ZrO2-based catalyst not only efficiently catalyzes the formation of N-hexylpentamide from valeronitrile and n-hexylamine but also the polymerization of 6-aminocapronitrile and ¿-caprolactam and does so with conventional as well as microwave heating. Microwave energy, however, heats the catalyst substantially, inducing selective heating that enhances the catalytic activity, compared to conventional heating. The drying behavior of (S)-N-acetylindoline-2-carboxylic acid with various moisture contents and of N-acetyl-(S)-phenylalanine, in a straightforward microwave-mediated drying setup, is presented in Chapter 6. The way energy is supplied to the system has a profound influence on the drying rate and on the internal temperature of the samples during drying. To achieve similar drying times with conventional heating as reached under microwave irradiation, extremely high energy inputs are required, causing extremely large temperature differences between the heating source and the sample. These results demonstrate that microwave energy is particularly useful for drying thermally unstable materials in short periods of time. Microwave heating is not a universally beneficial technique applicable to all reactions. The results we gathered suggest that every reaction has to be evaluated separately to judge whether microwave heating is a suitable upscaling tool and whether microwave heating is to be preferred over conventional heating

Self-contained in-vacuum in situ thin film stress measurement tool

A fully self-contained in-vacuum device for measuring thin film stress in situ is presented. The stress was measured by measuring the curvature of a cantilever on which the thin film was deposited. For this, a dual beam laser deflectometer was used. All optics and electronics needed to perform the measurement are placed inside a vacuum-compatible vessel with the form factor of the substrate holders of the deposition system used. The stand-alone nature of the setup allows the vessel to be moved inside a deposition system independently of optical or electronic feedthroughs while measuring continuously. A Mo/Si multilayer structure was analyzed to evaluate the performance of the setup. A radius of curvature resolution of 270 km was achieved. This allows small details of the stress development to be resolved, such as the interlayer formation between the layers and the amorphous-to-crystalline transition of the molybdenum which occurs at around 2 nm. The setup communicates with an external computer via a Wi-Fi connection. This wireless connection allows remote control over the acquisition and the live feedback of the measured stress. In principle, the vessel can act as a general metrology platform and add measurement capabilities to deposition setups with no modification to the deposition system

Phosphorus-based compounds for EUV multilayer optics materials

We have evaluated the prospects of phosphorus-based compounds in extreme ultraviolet multilayer optics. Boron phosphide (BP) is suggested to be used as a spacer material in reflective multilayer optics operating just above the L-photoabsorption edge of P (λ ≈9.2 nm). Mo, Ag, Ru, Rh, and Pd were considered for applications as reflector materials. Our calculations for multilayer structures with perfect interfaces show that the Pd/BP material combination suggests the highest reflectivity values, exceeding 70% within the 9.2 – 10.0 nm spectral range. We also discuss the potential of fabrication of BP-based multilayer structures for optical applications in the extreme ultraviolet rang

Improved burst pressure of LPCVD Si3N4 membranes by nanometer thick compressive adlayers

Si3N4 is a material widely used in MEMS technology. Its high mechanical strength makes Si3N4 attractive for applications where there is a need for ultrathin, yet robust, freestanding films, such as nanometer thick X-ray windows and support films for TEM. In this work, mechanical properties of Si3N4 and B-coated Si3N4 membranes were studied using a bulge test method. Burst pressure and corresponding membrane stress in Si3N4 layers were found to be significantly increased when a 3nm thick B layer was deposited on the top side of 25nm thick Si3N4 membranes, whereas a B layer applied to the bottom side of the membranes did not have an effect on the membrane strength. Using FEM simulations, we show that the B layer deposited at the top side decreases the maximum tensile stress in Si3N4 near the membrane edge, where a significant contribution to the total stress comes from bending. From this, we conclude that failure in single layer Si3N4 membranes during bulge test is dominated by fracture at the edge. The burst pressure of B-coated Si3N4 membranes was found to be higher for membranes with lower (more compressive) residual stress in B, which indicates that failure of bilayer membranes is caused by fracture initiated in the B layer. Please click Additional Files below to see the full abstract

Oxidation of thin film binary entropy alloys

In recent years, material science has put significant effort into understanding the behavior of multiple principle element alloys (MPEAs), notably the category high entropy alloys (HEAs). Most of these studies have been conducted on the micro to macro scales, yet the nanoscale remains relatively unexplored. Additionally, investigating the structural changes caused by amorphous oxidation for thin film MPEAs is particularly new, with no fundamental theory having been found. By studying ambient oxidation of thin film binary entropy alloys, we show how the Hume-Rothery rules affect the oxide formation and in particular the requirement of an increased temperature prior to the formation of polycrystalline oxides for these systems, compared to their single metal counterparts

Surface and sub-surface oxidation of thin films using Low Energy Ion Scattering

Ru and ZrN are candidate capping layers for applications such as catalysis, electronics and optical coatings: Ru exhibits a low resistivity, high thermal stability, excellent oxidation resistance and good diffusion capabilities. ZrN is thermally stable, and is known for its good mechanical properties. Although the oxidation process has been studied for both materials, the surface and especially the sub-surface oxidation is not properly understood and well addressed. We use the sub-monolayer surface sensitivity of the low energy ion scattering (LEIS) technique for in-situ monitoring of surface oxidation and determination of the oxygen sticking probabilities. From the LEIS in-depth signal, sub-nanometer sub-surface oxidation can be determined as a function of time and from these data oxygen diffusion constants can be extracted. These data support the applications for which adequate protecting surface films are required. i) Author to whom correspondence should be addressed. Electronic mail: [email protected]

Crystal surface characterization

Characterization of the structure of the crystal surface is essential for next generation electronics devices. Such as spin injection structures and topological insulators, to name a few. We have studied the advantages of characterization of the crystal surface based on the analysis of modulations of specular X-ray reflection occurred during the azimuthal scan in grazing incidence X-ray diffraction (GID) geometry