Fabrication and characterization of high-temperature microreactors with thin film heater and sensor patterns in silicon nitride tubes

In this paper the fabrication and electrical characterization of a silicon microreactor for high-temperature catalytic gas phase reactions, like Rh-catalyzed catalytic partial oxidation of methane into synthesis gas, is presented. The microreactor, realized with micromachining technologies, contains silicon nitride tubes that are suspended in a flow channel. These tubes contain metal thin films that heat the gas mixture in the channel and sense its temperature. The metal patterns are defined by using the channel geometry as a shadow mask. Furthermore, a new method to obtain Pt thin films with good adhesive properties, also at elevated temperatures, without adhesion metal is implemented in the fabrication process. Based on different experiments, it is concluded that the electrical behaviour at high temperatures of Pt thin films without adhesion layer is better than that of Pt/Ta films. Furthermore, it is found that the temperature coefficient of resistance (TCR) and the resistivity of the thin films are stable for up to tens of hours when the temperature-range during operation of the microreactor is below the so-called "burn-in" temperature. Experiments showed that the presented suspended-tube microreactors with heaters and temperature sensors of Pt thin films can be operated safely and in a stable way at temperatures up to 700 degrees C for over 20 h. This type of microreactor solves the electrical breakdown problem that was previously reported by us in flat-membrane microreactors that were operated at temperatures above 600 degrees C.     

Humic acid fouling during microfiltration

A major factor limiting the use of microfiltration for surface water treatment is membrane fouling by natural organic matter. The extent and mechanisms of humic acid fouling during microfiltration have been examined using stirred cell filtration experiments and scanning electron microscopy. The extent of fouling was strongly dependent on both the source and preparation of the humic acid solutions. The large flux decline observed during constant pressure microfiltration was caused by the formation of a humic acid deposit located on the upper surface of the membrane. Prefiltration of the humic acid solutions dramatically reduced the rate of fouling through the removal of large humic acid aggregates. The initial fouling in this system was determined almost entirely by the convective deposition of these large particles/aggregates on the membrane surface. This initial deposit accelerated the subsequent rate of humic acid fouling, possibly serving as a nucleation site for deposition of macromolecular humic acids.     

Microreactors as the new way of intensification of heterogeneous processes

Microstructure reactors (microreactors) in recent decades became one of the most actively studied subjects of the reaction equipment aimed at intensification of chemical processes and increase in their safety. It is not surprising because due to miniature dimensions of microstructures which do not exceed 2 mm microreactors contribute to minimization of the material at their production as well as raw material and energy in the process of exploitation. Moreover, due to acceleration of heat and mass transfer the productivity of equipment with microreactors in a range of cases is significantly higher than classical batch reactors applied in industry. The brief overview of the modern development and achievements of microreactor technology is given in this article by an example of heterogeneous reaction systems which are different by their nature and occur in different types of microreactors: phase-transfer catalysis, biocatalysis, and synthesis of nanoparticles. A special attention in the article is paid to the aspects of intensification of the considered processes because exactly the possibility of intensification makes microreactor technology attractive for the industry.     

Teflon-coated silicon microreactors: impact on segmented liquid-liquid multiphase flows

We describe fluoropolymer modification of silicon microreactors for control of wetting properties in chemical synthesis applications and characterize the impact of the coating on liquid-liquid multiphase flows of solvents and water. Annular flow of nitrogen gas and a Teflon AF (DuPont) dispersion enable controlled evaporation of fluoropolymer solvent, which in turn brings about three-dimensional polymer deposition on microchannel walls. Consequently, the wetting behavior is switched from hydrophilic to hydrophobic. Analysis of microreactors reveals that the polymer layer thickness increases down the length of the reactor from ～1 to ～13 μm with an average thickness of ～7 μm. Similarly, we show that microreactor surfaces can be modified with poly(tetrafluoroethylene) (PTFE). These PTFE-coated microreactors are further characterized by measuring residence time distributions in segmented liquid-liquid multiphase flows, which display reduced axial dispersion for the coated microreactors. Applying particle image velocimetry, changes in segment shape and velocity fluctuations are observed resulting in reduced axial dispersion. Furthermore, the segment size distribution is narrowed for the hydrophobic microreactors, enabling further control of residence distributions for synthesis and screening applications.     

Physicochemical processes in polymer materials upon exposure at the MIR orbital space station

An overview of papers on the investigation of physicochemical processes occurring in polymer (polyimide, fluoropolymer, fluoropolyimide, unilaterally aluminized polyimide) films during long-term exposure at the Mir orbital space station is presented in the article dedicated to the centenary of the birthday of Professor S. Ya. Pshezhetskii. Anisotropic nano- and micro-structures oriented in the direction of motion of the station have been detected for the first time with the scanning electron microscopy and atomic force microscopy techniques on surfaces of films exposed to the effects of all factors of open space. The formation of anisotropic structures has been confirmed by the investigation of the light scattering circular diagrams and surface properties of the films. It is supposed that the formation of such structures results from collisions of a flux of oxygen atoms having an energy of ∼5 eV, with the outer surface of the films. It has been found with the use of electronic absorption and infrared spectroscopy that the changes in the surface and optical properties of the films are caused by the occurrence of oxidation processes, degradation of macromolecules, and opening of the imide cycles and aromatic rings of polyimide. Unilaterally aluminized polyimide films are the least resistant to the effects of outer space factors, what can be explained by the photoelectron mechanism of polymer destruction.     

Fouling in direct contact membrane distillation process

The formation of deposit on the membrane surface (fouling) is one of the major operating problems of membrane distillation process. The influence of fouling on the performance of this process was investigated during the concentration of wastewater with proteins, bilge water, brines, and the production of demineralized water. The experiments were performed with polypropylene capillary membranes. The morphology and composition of the fouling layer were studied using Fourier transform infrared with diffuse reflectance spectroscopy and scanning electron microscopy coupled with the energy dispersing spectrometry. Fouling with various intensity was observed in most of the studied cases. Permeate flux decline was mainly caused by an increase in the heat resistance of the fouling layer. However in the case of non-porous deposit, a magnitude of the permeate flux was also determined by a resistance of water transport through the deposit layer. It was found the deposits were formed not only on the membrane surface, but also inside the pores. Salt crystallization in the membrane pores besides their wetting, also caused the mechanical damage of the membrane structure. The intensity of the fouling can be limited by the pretreatment of feed and a selection of the operating conditions of membrane distillation.     

Membrane fouling in a submerged membrane bioreactor (MBR) under sub-critical flux operation: Membrane foulant and gel layer characterization

Membrane foulants and gel layer formed on membrane surfaces were systematically characterized in a submerged membrane bioreactor (MBR) under sub-critical flux operation. The evaluation of mean oxidation state (MOS) of organic carbons and Fourier transform infrared (FT-IR) spectroscopy demonstrated that membrane foulants in gel layer were comprised of not only extracellular polymeric substances (EPS) (proteins, polysaccharides, etc.) but also other kinds of organic substances. It was also found that fine particles in mixed liquor had a strong deposit tendency on the membrane surfaces, and membrane foulants had much smaller size than mixed liquor in the MBR by particle size distribution (PSD) analysis. Gel filtration chromatography (GFC) analysis showed that membrane foulants and soluble microbial products (SMP) had much broader distributions of molecular weight (MW) and a larger weight-average molecular weight (M_w) compared with the influent wastewater and the membrane effluent. Scanning electron microscopy (SEM) and energy-diffusive X-ray (EDX) analysis indicated that membrane surfaces were covered with compact gel layer which was formed by organic substances and inorganic elements such as Mg, Al, Fe, Ca, Si, etc. The organic foulants coupled the inorganic precipitation enhanced the formation of gel layer and thus caused membrane fouling in the MBR.     

Polymeric membranes: Surface modification for minimizing (bio)colloidal fouling

This paper presents an overview on recent developments in surface modification of polymer membranes for reduction of their fouling with biocolloids and organic colloids in pressure driven membrane processes. First, colloidal interactions such as London–van der Waals, electrical, hydration, hydrophobic, steric forces and membrane surface properties such as hydrophilicity, charge and surface roughness, which affect membrane fouling, have been discussed and the main goals of the membrane surface modification for fouling reduction have been outlined. Thereafter the recent studies on reduction of (bio)colloidal of polymer membranes using ultraviolet/redox initiated surface grafting, physical coating/adsorption of a protective layer on the membrane surface, chemical reactions or surface modification of polymer membranes with nanoparticles as well as using of advanced atomic force microscopy to characterize (bio)colloidal fouling have been critically summarized.     

ALD resist formed by vapor-deposited self-assembled monolayers

A new process of applying molecular resists to block HfO2 and Pt atomic layer deposition has been investigated. Monolayer films are formed from octadecyltrichlorosilane (ODTS) or tridecafluoro-1,1,2,2-tetrahydrooctyltrichlorosilane (FOTS) and water vapor on native silicon oxide surfaces and from 1-octadecene on hydrogen-passivated silicon surfaces through a low-pressure chemical vapor deposition process. X-ray photoelectron spectroscopy data indicates that surfaces blocked by these monolayer resists can prevent atomic layer deposition of both HfO2 and Pt successfully. Time-dependent studies show that the ODTS monolayers continue to improve in blocking ability for as long as 48 h of formation time, and infrared spectroscopy measurements confirm an evolution of packing order over these time scales.     

Hydrogenated polycrystalline diamond films: Elastic and inelastic electron reflectivity

The microstructure and properties of carbon-based thin films depend on the deposition process and conditions used, including pressure, gas phase composition, and substrate temperature, as well as the energy of the reactive species (atoms or ions). For instance concerning diamond films, each method results in different type of films which may differ in terms of diamond grain size (from nano to micro), grain boundary nature, hydrogen content, defect density, amorphous or graphitic components, morphological properties and different chemical and physical properties. Among them, the well-known negative electron affinity, very attractive for the detection and emission of electrons, and high conductivity of diamond surfaces are properties of fully hydrogenated diamond surfaces. Similarly, diamond grain size may influence the electronic and optical properties of the films. More generally the chemical and physical characterization of the uppermost surface atomic layer of diamond films presents a great challenge.In this review we present results on hydrogen bonding configuration in hydrogenated polycrystalline diamond films of varying size (few nanometers up to micrometers) obtained by high resolution electron energy loss spectroscopy (HREELS). More precisely we will present energy loss spectra extended up to 800 meV, as well as elastic and inelastic reflectivity curves (associated to different vibrational modes of hydrogenated diamond surfaces), measured over the 3–18 eV electron energy range. We will show in particular that due to the specific features of diamond bulk electronic band structure, which is maintained up to the surface in the case of fully hydrogenated diamond, it is possible to extract from these data valuable information about the surface properties and composition such as diamond or graphitic like nature of the films, surface versus lattice nature of the vibrational modes.     

Effects of oxygen plasma on cellulose surface

These studies aimed to investigate in detail changes on cellulose surfaces treated with low pressure oxygen plasma at various exposure times. Modifications of cellulose films were studied in respect to topography effects by means of atomic force microscopy and scanning electron microscopy. Chemical effects of plasma treatment were studied using X-ray photoelectron spectroscopy and X-ray diffractometry. Results show that the topographical evolution of the surfaces to rougher ones is not at all gradual. Local maxima of fractionation and the surface size regularity were investigated using surface fractal analysis and Wenzel roughness factors, respectively. It was shown, that plasma treatments decompose the cellulose material by formation of highly functionalized molecules. Such plasma-initiated and supported reactions taking place on the sample surface. The bulk phase and in particular, the crystalline domains are not influenced by plasma treatments. The studies provide useful information to understand the plasma reaction on amorphous and crystalline regions of cellulose surfaces and allow to predict effects of the plasma treatment on physical and chemical properties of much more complex cellulose systems such as cotton fibres and fabrics.     

Durable antifog films from layer-by-layer molecularly blended hydrophilic polysaccharides

Mechanically durable, long-lasting antifog coatings based on polysaccharides were developed using a layer-by-layer (LBL) assembly process. The unique properties of these coatings are a result of a molecular-level blending of the polysaccharides, with multilayers containing chitosan and carboxymethyl cellulose providing the best overall properties. The antifog properties resulted from a strong interaction between the polar and H-bonding elements of the assembled polymers and water molecules and the concomitant formation of thin films of water. Environmental scanning electron microscopy (ESEM) studies confirmed that fogging coatings are decorated with light scattering, micrometer-sized droplets of water whereas antifogging coatings remain droplet free. To improve the mechanical durability of the multilayer films on substrates, the surface was modified via self-assembly of epoxy-functionalized silane molecules. Cross-linking chemistry was then applied to improve the mechanical robustness of the LBL films on various surfaces. These films were characterized using several techniques: optical profilometery (PL), spectroscopic ellipsometry (EL), contact angle goniometry (CA), and atomic force microscopy (AFM). The antifog properties of the films were evaluated by several tests under different environmental conditions. This work demonstrates that the unique water-adsorbing properties of polysaccharides can be exploited to create permanent antifog properties, which may be useful for various applications.     

Development of a multireactor microfluidic system for the determination of DNA using real-time polymerase chain reaction

A microfluidic system that allowed us to perform the real-time polymerase chain reaction (PCR) in a glass-silicon microchip containing nine 250-nL microreactors was developed and studied. The resulting high heating/cooling rates of a PCR mixture in a microreactor allowed us to optimize the amplification mode (1 min/cycle). The silicon surface of microreactors was successfully passivated. The resulting analytical system allowed us to measure the PCR kinetic curves in chip microreactors at a DNA concentration of ～5 × 10⁴ copies per microreactor. It was found that, if the PCR is performed in a microchip with real-time detection using the optimized amplification mode, the result can be obtained 13–14 min after the onset of reaction.     

Surface hydration and its effect on fluorinated SAM formation on SiO2 surfaces

Substrate hydration is demonstrated to be crucial to film quality during self-assembled (SA) film deposition of tridecafluoro-1,1,2,2,-tetrahydrooctyltrichlorosilane (FOTS) from the vapor phase. The surface hydration was studied by thermogravimetric analysis, and a model was developed to predict the conditions necessary to desorb all of the water adsorbed on a fused silica surface without significantly altering the concentration of the surface hydroxyl groups. The nature of the SA film was investigated as a function of the degree of rehydration of the dehydrated silica surface. The wettability and microstructure of the SA films were examined by water contact angle, ellipsometry, X-ray photoelectron spectroscopy, and atomic force microscopy. There is an optimum degree of substrate hydration, on the order of 1-1.2 monolayers of adsorbed water, required to produce a dense, durable and uniform FOTS film with high water repellency and a smooth surface.     

Physico-chemical study of fouling mechanisms of ultrafiltration membrane on Biwa lake (Japan)

Many studies have been undertaken to understand the fouling of the ultrafiltration membranes in drinking water treatment. Physico-chemical fouling of membranes depends on characteristics of the raw water and membrane surface properties. In the case of Biwa lake, some chemical parameters as Si and Fe concentrations change with temperature (season) causing irreversible fouling. While some exits on the influence of the particle mineralogy on the fouling, little work has been developed to elucidate the relation between the physicochemical complexity of the cake and the fouling. Generally clays or oxides are known to lead to a reversible fouling. In this work, the interactions between a UF organic membrane with minerals leading to a hardly reversible fouling are studied. In the case of the Biwa lake water, fouling of ultrafiltration membranes results from the formation of a Si-rich ferric gel directly deposited on the membrane surface and a secondary allophanic gel layer at a bigger distance. The deposit nature and the membrane/cake interactions were studied using infra-red, X-ray diffraction, Al and Si NMR and EXAFS technics. The effect of mineral particles, especially ferric oxides associated with silica, has been demonstrated. The formation of Fe---Si gel directly on the membrane surface is mainly responsible for the fouling. The change of these particles is less negative than the membrane surface. The structure of such a material is complex. The low permeability of the gel is at the prime origin of the fouling.     

Effects of ultrafiltration membrane surface properties on Pseudomonas aeruginosa biofilm initiation for the purpose of reducing biofouling

Biofilm fouling is one of the major obstacles hindering the use of membranes in water processing systems. There are a series of events that take place during biofilm formation, one of the most interesting and important issues of biofouling is the initial attachment of microorganisms to the surface. Therefore, effects that surface properties have on biofilm fouling are important to attachment and were examined. Hydrophobicity, surface charge and roughness were measured for several polymeric surfaces of interest in water processing membrane systems. These surfaces were then subjected to conditioning layer formation and biofilm fouling, both of which were quantified. The results show that biofilm initiation by a strain of Pseudomonas aeruginosa increases as the surface becomes more rough and more hydrophobic, while fouling is minimal when surface charge is minimized and increases with increasing charge, whether positive or negative.     

Wood surface protection with different alkoxysilanes: a hydrophobic barrier

This paper describes coatings on wood surfaces made by dipping the wood into solutions of different alkoxysilanes. The silanes used as precursors contain different organic groups [R’Si(OR’’)]. These materials tend to deposit as inorganic–organic polymeric films, where the organic groups (aliphatic hydrocarbons, fluorinated hydrocarbons or aromatic substituents) show hydrophobic properties, which reduce the wettability of the surface. The effects of these treatments on the wood surface were extensively studied using various analytical techniques: scanning electron microscopy with energy dispersive X-ray spectrometry, Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, water contact angle measurements, and flame resistance tests. The resulting data show that the chemical treatment changes the wood’s surface energy, reducing its wettability and reaction to fire. The main innovative finding of this research is that the coatings obtained from a cheaper precursor have a similar performance to that of the more expensive precursors normally used.     

NHS-ester functionalized poly(PEGMA) brushes on silicon surface for covalent protein immobilization

Poly(PEGMA) homopolymer brushes were developed by atom transfer radical polymerization (ATRP) on the initiator-modified silicon surface (Si-initiator). Through covalent binding, protein immobilization on the poly(PEGMA) films was enabled by further NHS-ester functionalization of the poly(PEGMA) chain ends. The formation of polymer brushes was confirmed by assessing the surface composition (XPS) and morphology (atomic force microscopy (AFM), scanning electronic microscopy (SEM)) of the modified silicon wafer. The binding performance of the NHS-ester functionalized surfaces with two proteins horseradish peroxidase (HRP) and chicken immunoglobulin (IgG) was monitored by direct observation. These results suggest that this method which incorporates the properties of polymer brush onto the binding surfaces may be a good strategy suitable for covalent protein immobilization.     

Surface grafted sulfobetaine polymers via atom transfer radical polymerization as superlow fouling coatings

One of the sulfobetaine methacrylate (SBMA) monomers, N-(3-sulfopropyl)-N-(methacryloxyethyl)-N,N-dimethylammonium betaine, was polymerized onto initiator-covered gold surfaces using atom transfer radical polymerization (ATRP) to form uniform polymer brushes. Self-assembled monolayers (SAMs) with ATRP initiators were characterized by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The thickness of grafted poly(SBMA) films was measured by ellipsometry. Fibrinogen adsorption on poly(SBMA) grafted surfaces was measured with a surface plasmon resonance (SPR) sensor. Two approaches were compared to graft ATRP initiators onto gold surfaces for surface polymerization and subsequent protein adsorption on these polymer grafted surfaces. The first was to prepare a SAM from omega-mercaptoundecyl bromoisobutyrate onto a gold surface. Superlow fouling surfaces with well-controlled poly(SBMA) brushes were achieved using this approach (e.g., fibrinogen adsorption <0.3 ng/cm2). The second approach was to react bromoisobutyryl bromide with a hydroxyl-terminated SAM on a gold surface. Although protein adsorption decreased as the density of surface initiators increased, the surface prepared using the second approach was not able to achieve as low protein adsorption as the first approach. Key parameters to achieve superlow fouling surfaces were studied and discussed.     

Fouling of reverse osmosis membranes by biopolymers in wastewater secondary effluent: Role of membrane surface properties and initial permeate flux

Reverse osmosis (RO) is being increasingly used in treatment of domestic wastewater secondary effluent for potable and non-potable reuse. Among other solutes, dissolved biopolymers, i.e., proteins and polysaccharides, can lead to severe fouling of RO membranes. In this study, the roles of RO membrane surface properties in membrane fouling by two model biopolymers, bovine serum albumin (BSA) and sodium alginate, were investigated. Three commercial RO membranes with different surface properties were tested in a laboratory-scale cross-flow RO system. Membrane surface properties considered include surface roughness, zeta potential, and hydrophobicity. Experimental results revealed that membrane surface roughness had the greatest effect on fouling by the biopolymers tested. Accordingly, modified membranes with smoother surfaces showed significantly lower fouling rates. When Ca²⁺ was present, alginate fouled RO membranes much faster than BSA. Considerable synergistic effect was observed when both BSA and alginate were present. The larger foulant particle sizes measured in the co-existence of BSA and alginate indicate formation of BSA-alginate aggregates, which resulted in greater fouling rates. Faster initial flux decline was observed at higher initial permeate flux even when the flux was measured against accumulative permeate volume, indicating a negative impact of higher operating pressure.