E. Räuchle

Proton-conducting polymers with reduced methanol permeation

The permeability of Nafion® 117 and some types of acid-base and covalently crosslinked blend membranes to methanol was investigated. The methanol crossover was measured as a function of time using a gas chromatograph with a flame ionization detector. In comparison to Nafion, the investigated acid-base and covalently crosslinked blend membranes show a significant lower permeation rate to methanol. Additionally, another method to reduce the methanol permeability is presented. In this concept a thin barrier layer is plasma polymerized on Nafion 117 membranes. It is shown that a plasma polymer layer with a thickness of 0.3 μm reduces the permeability to methanol by an order of magnitude.

Improvement of the adhesion of low-energy polymers by a short-time plasma treatment

Abstract Foils of polyethylene, polypropylene and polyethylene terephthalate are exposed to low-pressure plasmas. The plasma is produced by microwaves ( f =2.45 GHz) at the electron cyclotron resonance, with electron densities of about 10 11 cm -3 and electron temperatures between 2 and 4 eV. Oxygen and hydrogen are used here for plasma treatment at a pressure of typically 3 Pa. Quartz-like and polyethylene-like thin films are produced on the polymers as adhesion agents. The adhesion power between the polymers and the adhesive is measured by pull-off tests. The adhesion strongly increases by a plasma treatment, for polyethylene by a factor of 20 after treatment of a few seconds. The surface tension of the polymer foils is measured using the Cahn surface tensiometer. It is shown that the polar part of the surface tension of the tested polymers increases with plasma treatment, whereas the dispersion part is only weakly influenced. The adhesion power and the polar part of the surface tension of the polymers are correlated. The polymer surfaces are investigated by scanning electron microscopy and by infrared spectroscopy.

Duo-Plasmaline — a linearly extended homogeneous low pressure plasma source

Abstract A linearly extended plasma source — the Duo-Plasmaline — was developed. It can be used in a large frequency and pressure range. The plasma is excited by microwaves of 2.45 GHz at a pressure The plasma parameters were measured in dependence on the axial and the radial distances from the plasma source: electron density and electron temperature by a Langmuir probe, microwave intensity by a pick up probe. It is shown that an excellent homogeneity of the electron density can be achieved along the axis of the plasma source. In the radial distance the electron density decreases nearly exponentially. A further result is that the microwave field is radially shielded by the electron density of 2×1011 cm−3 produced near by the surface of the quartz tube. The etch rate of polymethylmethacrylate (Plexiglas®) in an oxygen plasma and the deposition of quartz-like films produced in a plasma of hexamethyldisiloxane and oxygen as monomers show also an excellent axial homogeneity. This newly developed plasma source is well suited for large area plasma treatment, e.g. coating, etching, cleaning and surface modification. Two-dimensional plasma devices can be obtained by an array of individual plasma sources of the described type.

Barrier properties of plasma-polymerized thin films

Abstract Thin hydrocarbon films are deposited on Nafion™ membranes in a low-pressure plasma excited by microwaves. Gas mixtures of hexane (C 6 H 14 ) with hydrogen (H 2 ) were used as monomers. The permeability of methanol through the Nafion membranes modified by plasma polymer films is investigated as a function of the C 6 H 14 /H 2 ratio of the gas mixture. The methanol permeability was measured as a function of time using a gas chromatograph with a flame ionisation detector. It is shown that a plasma polymer film reduces the permeability of methanol by a factor of about 15.

Plasma polymerized barrier films on membranes for direct methanol fuel cells

The methanol permeation through fuel cell relevant Nafion® membranes is investigated at different concentrations of methanol in aqueous solutions. Thin plasma polymerized barrier films are deposited on Nafion® membranes in a low pressure microwave generated plasma to reduce their methanol permeability. The methanol permeability was measured as a function of time using a gas chromatograph with a flame ionisation detector. It is shown that a plasma polymer layer with a thickness of approximately 0.27 μm on Nafion® membranes reduces the permeability to methanol by a factor of approximately 20.

Linearly extended plasma source for large-scale applications

Abstract A linearly extended plasma source — the Duo-Plasmaline — is characterised. The plasma is excited by microwaves of 2.45 GHz in a pressure range between 5 and 500 Pa. The device is similar to a coaxial wave guide. A quartz tube conveys through a vacuum chamber. A copper rod centered in the quartz tube is the inner conductor, and the plasma produced in the low-pressure regime outside the quartz tube represents the outer conductor. The microwaves are fed from both ends of the quartz tube into the wave guide and propagate mainly along the inner rod and the tube, filled with air at atmospheric pressure. The device generates a linearly extended plasma up to several meters, mainly controlled by the pressure and microwave power. The plasma source used here is expanded by a second parallel quartz tube both mounted and supplied parallel as a double line system. A sample stage movable perpendicular to the plasma source was mounted. The plasma was characterised for different plasma conditions by measurements of the electron density in relation to the axial and radial distances from the plasma source. The excellent axial homogeneity of the electron density is reflected in the homogeneous axial distribution of the etch rate of polymethylmetacrylate in an oxygen plasma. Also, the axial homogeneity of the deposition rate of quartz-like films polymerised in a plasma from hexamethyldisiloxane (HMDSO) and oxygen, is demonstrated. The newly designed linear plasma source is well suited for large-area plasma treatment and coating.

XPS and IR analysis of thin barrier films polymerized from C2H4/CHF3 ECR‐plasmas

Thin fluorocarbon polymer films are prepared on PE-foils in low-pressure electron cyclotron resonance plasmas using ethylene (C2H4) and trifluoromethane (CHF3) as monomers. The thin fluorinated hydrocarbon layers strongly reduces the permeability of polyethylene to alkanes. For example, the permeation of toluene was decreased by a factor of about 100 by a single, thin fluorocarbon layer. A further reduction of the permeation down to a factor of 1600 can be obtained by a multilayer coating. X-ray photoelectron spectroscopy and Fourier transform IR spectroscopy are used to characterize the plasma polymerized films. It is shown that the addition of CHF3 to a C2H4 plasma leads to an increase of CF3—, CF2—, and CF— groups and to a decrease of CH3— and CH2— groups in the film. The chemical composition of the polymer layers and their toluene permeabilities are discussed.

Gigatron® — a new source for low-pressure plasmas

Abstract A linearly extended microwave plasma source was developed that is used in the pressure range below 1 kPa. In the Gigatron® device, the microwaves (f=2.45 GHz) are coupled into the vacuum chamber in the form of a coaxial waveguide, consisting of an inner copper rod and a surrounding quartz tube with atmospheric pressure between them. The plasma is produced outside the quartz tube and forms the outer conductor of a coaxial waveguide. The homogeneity and the extension of the plasma were investigated with respect to process parameters as a function on the radial and axial distances from the plasma source. It is shown that the plasma can be extended to a length of more than 1 m for an applied microwave power of 1.2 kW.

Influence of plasma surface treatment on the adhesion of thin films on metals

Abstract Thin quartz-like polymer films are prepared in a low pressure electron cyclotron resonance plasma operated at 2.45 GHz. Hexamethyldisiloxane is used as monomer in a mixture with oxygen or hydrogen. The adhesion of the polymer films on metal surfaces is studied by measuring the adhesion power with an adhesion test instrument. It is shown that a plasma surface pre-treatment is necessary to obtain good film adhesion. The deposited films are characterized by their IR spectrum measured by the Fourier transform IR technique. The adhesion power depends on the chemical nature of the film; in some cases the surface energy determined by the test ink method correlates with the adhesion power.

A new linearly extended bifocal microwave plasma device

Abstract A new kind of linearly extended plasma source was developed. The prototype of such a device was built up as a barrel with elliptical cross-section. This bifocal shape has two focus lines. A linearly extended microwave antenna is placed along one focus line and the sector of plasma treatment is placed along the second focus line. The antenna is a linear emitter of radial divergent microwave radiation. In this work, a frequency of 2.45 GHz in pulsed and continuous wave (CW) mode was used. The power up to 6 kW CW and 10 kW pulsed is adjustable, pulse on/off times are variable in a wide field. The microwave is reflected by the metallic walls of the barrel and concentrated in the second focus line apart from the antenna. There a quartz tube is placed, that can be evaporated and filled with a working gas at a pressure between 10 2 and 10 5 Pa. The presented experiments have been done with Argon up to atmospheric pressure at a gas flow up to 6 slm.