Publications Search Results
1 - 9 of 9
-
PublicationInfluence of Sintering Additives on Modified (Ba,Sr)(Sn,Ti)O3 for Electrocaloric Application( 2023)
-
PublicationAir plasma treatment of aluminium trihydrate filled poly(methyl methacrylate)( 2018)Air plasma treatments of aluminium trihydrate filled poly(methyl methacrylate) polymer (PMMA) composites were carried out in a dielectric barrier discharge. X-ray photoelectron spectroscopy (XPS) and attenuated total reflection - infrared spectroscopy have been employed to analyse the changes in chemical composition. Confocal laser-scanning microscopy and atomic force microscopy yielded the impact on surface structure and morphology. Surface free energy (SFE) measurements and adhesive bond strength tests were used to correlate the results to possible applications like gluing and lacquering. The plasma treatments resulted mainly in an etching of the matrix polymer and a minor chemical modification. An apparent contradiction of XPS, ATR and SFE results was attributed to a re-polymerization and re-deposition of the etched PMMA material as debris back onto the surface. This effect, which is seldom taken into account, annihilated the positive impact of the plasma treatment due to the debris acting as rated break point.
-
PublicationImpact of electrode design, supply voltage and interelectrode distance on safety aspects and characteristics of a medical DBD plasma source( 2013)In the frame of plasma source development for dermatological applications in the field of plasma medicine, operational safety of the devices is of superior priority. For sources based on the concept of dielectric barrier discharges (DBD), electric potentials with amplitudes in the range of some kV are arranged in close proximity to the skin of patients, wherein dielectric strength of the electrodes and leakage currents are crucial for electrical applicability. In this work, ceramic electrodes of 10 mm in diameter and varying ceramic thickness are operated at input powers up to 300 mW against non-biological counter electrodes. In a combined experimental and numerical approach, electric fields inside the ceramic are determined, whereas values are well below the dielectric strength of the material. The spectrally weighted plasma emission is within limit values of exposure to human skin as long as daily treatment does not exceeded 7 h. Neutral gas temperatures of up to 310 K are determined which underline the minor thermal impact of the plasma exposure. In contrast, values for reduced electric fields are of the order of some hundred Townsend and thus the electrons can initiate various secondary effects such as chemical reaction chains. Consequently, ozone concentrations in the discharges are quantified between 230 ppm and 1140 ppm in close proximity to the actual discharge volume and the results are discussed in the frame of risk assessment for therapeutic applications in dermatology.
-
PublicationComparison of the microstructural characteristics and electrical properties of thermally sprayed Al2O3 coatings from aqueous suspensions and feedstock powders( 2012)In this work the microstructural characteristics and electrical insulating properties of thermally sprayed alumina coatings produced by suspension-HVOF (S-HVOF) and conventional HVOF spray processes are presented. The electrical resistance at different relative air humidity (RH) levels (from 6 to 97% RH) and values of dielectric strength were investigated by direct current electrical resistance measurements, electrochemical impedance spectroscopy, and dielectric breakdown tests. Relationships between electrical properties and coating characteristics are discussed. At low humidity levels (up to 40% RH) the electrical resistivities of S-HVOF and HVOF coatings were on the same order of magnitude (10(exp 11) Omega m). At a very high humidity level (97% RH) the electrical resistivity values for the S-HVOF coatings were in the range 10 (exp 7)-10(exp 11) Omega·m, up to five orders of magnitude higher than those recorded for the HVOF coating (orders of magnitude of 10(exp 6) Omega·m). The better electrical resistance stability of the suspension-sprayed Al 2O 3 coatings can be explained by their specific microstructure and retention of a higher content of alpha-Al 2O 3. The dielectric strength E d of suspension-sprayed coatings was found to be 19.5-26.8 kV·mm -1 for coating thicknesses ranging from 60 to 200 µm. These values were slightly lower than those obtained for conventional HVOF coatings (up to 32 kV·mm -1). However, it seemed that the dielectric strength of conventionally sprayed coatings was more sensitive to the coating thickness (when compared with the values of E d determined for S -HVOF coatings) and varied to a greater extent (up to 10 kV·mm -1) when the coating thickness varied in the range 100-200 µm.
-
PublicationInorganic-organic hybrid materials (ORMOCER®s) for multilayer technology - passivation and dielectric behavior( 2002)Inorganic-organic hybrid polymers (ORMOCER(R)s) have been synthesized by sol-gel processing. The materials can be functionalized such that their physical and chemical properties can be reproducibly tailored towards the desired application, i.e., electronics, optics (passives/actives) or passivation technology. Besides, the materials which show negative resist behavior, can be patterned by UV exposure with good resolution. The materials are well-suited for thin and thick film technology using conventional coating technology, applied in multi-layer technology. We here particularly focus on materials for passivation against environmental influences, which additionally exhibit very good dielectric properties.
-
PublicationImproved current-voltage characteristics of downstream plasma enhanced chemical vapor deposition SiNx deposited at low temperature by using He as a dilution gas( 1997)We have compared the current-voltage characteristics of silicon nitrides prepared from the two gas combinations N2/NH3/SiH4 (N2-SiNx) and He/NH3/SiH4 (He-SiNx) at temperatures between 100 and 350 degrees C. A downstream plasma enhanced chemical vapor deposition reactor with a non-ECR microwave plasma source has been used. While N2-SiNx with reasonable electrical properties requires deposition temperatures of about 350 degrees C, the characteristics of He-SiNx even improve at decreasing process temperatures. Almost identical current-voltage characteristics are found for both N2-SiNx and He-SiNx prepared at 350 degrees C exhibiting an Ohmic behavior at low fields and a Poole-Frenkel (PF) conduction at high fields. At a deposition temperature of 100 degrees C the He-SiNx with a dielectric strength of 1.13*107 V/cm and an onset field strength of the PF conduction of 6.9*106 V/cm is in contrast to the N2-SiNx with a dielectric strength of 4.3*106 V/cm and a PF onset field strength of as low as 2*105 V/cm resulting in many orders of magnitude higher current flow. In order to find a correlation between the dielectric and the structural properties of the silicon nitrides several analyses are performed. The He-SiNx proves to be superior to the N2-SiNx concerning refractive index, mass density and buffered HF etch rate at every deposition temperature and the deterioration with decreasing deposition temperature is weaker. All SiNx films are found to be nitrogen-rich at a N:Si ratio of 5:3. As expected, the hydrogen content increases with decreasing deposition temperature, being lower for the He-SiNx than for the N2-SiNx. In all layers most of the hydrogen is bonded to the nitrogen atoms. A simple model is proposed that explains the difference between the two kinds of low-temperature SiNx by the microscopic void structure of the material.
-
PublicationInvestigation of high-field conductivity and dielectric strength of nitrogen containing polycrystalline diamond films( 1995)The influence of nitrogen doping on the electrical properties of polycrystalline diamond films has been studied. The films were prepared in a microwave plasma chemical vapor deposition process using a H2/CH4/N2 gas mixture. The CH4 concentration was held constant at 0.5 per cent and the nitrogen to carbon atomic ratio was varied between 0.01 and 0.2. The phase purities, surface morphologies, and the nitrogen contents of the films were analyzed by Raman spectroscopy, scanning electron microscopy, and secondary-ion mass spectroscopy, respectively. From current-voltage characteristics at field strengths up to 10(exp 6) V cm(exp - 1) and in the temperature range between 300 and 800 K the conductivity was determined. The dielectric strength was obtained from the breakdown voltage measured using a voltage ramping rate between 50 and 100 V s(exp - 1). A minimum in high field and high-temperature conductivity and a maximum in dielectric strength was found for the samples prepared with a nitrog en to carbon atomic ratio of 0.02. Compared with nitrogen free samples the conductivity is lowered by more than three orders of magnitude and the dielectric strength is enhanced by a factor of two. The results will be discussed in terms of compensation of acceptor states by nitrogen donors and structural changes of the films.
-
PublicationInvestigation of the high-field conductivity and dielectric strength of nitrogen containing polycrystalline diamond films( 1994)The influence of nitrogen doping on the electrical properties of polycrystaline diamond films has been studied. The films were prepared in a microwave plasma CVD process usind a H2/CH4/N2 gas mixture. The CH4 concentration was held constant at 0.5% and the nitrogen to carbon automatic ratio was varied between 0.01 and 0.2. The phase purities, surface morphologies and the nitrogen contents of the films were analysed by Raman spectroscopy, scanning electron microscopy, and secondary-ion mass spectroscopy, respectively. From current-voltage characteristics at field strengths up to 10 high 6 V cm high -1 and the temperature range between 300 K and 800 K the conductivity was determined. The dielectric strength was obtained from the breakdown voltage measured using a voltage ramping rate between 50 V sec high -1 and 100 V sec high -1. A minimum in high field and high temperature conductivity and a maximum in dielectric strength were found for the samples prepared with a nitrogen to carbon at omic ratio of 0.02. Compared with nitrogen free samples the conductivity is lowered by more than three orders of magnitude and the dielectric strength is enhanced by a factor of two. The results will be discussed in terms of compensation of acceptor states by nitrogen donors and structural changes of the films.
-
PublicationOptimized poling of nonlinear optical polymers based on dipole-orientation and dipole-relaxation studies( 1994)Nonlinear optical polymers contain molecular dipoles with very large hyperpolarizabilities in a glassy polymer matrix. Two typical examples-a guest-host system with dispersed polar dye molecules and a side-chain material with chemically attached molecular dipoles-were investigated by means of poling experiments, dielectric spectroscopy, thermally stimulated depolarization, and electro-optical thermal analysis. The dielectric behavior of both polymers can be described by the phenomenological Havriliak-Negami equation, and the existence of master curves for both materials demonstrates the validity of the time-temperature superposition principle above the respective glass transitions. Temperature-dependent mean relaxation times and relaxation-time distributions calculated from the dielectric data allow for an optimization of poling times. The dielectric relaxation strengths obtained from poling current and field, from dielectric measurements, and from thermally stimulated depolarization are in very good agreement and thus represent a useful measure of the polarization in poled polymers. From the temperature dependence of the polarization, optimal poling temperatures may be derived. Electro-optical thermal analysis yields the same temperature-stability curves as thermally stimulated depolarization and is therefore a valuable tool for investigating the stability of poled polymers, especially since it is not sensitive to charge effects. Optimal poling fields and currents must be selected as a compromise between high dipole mobilities (short relaxation times) and low bulk conductivities.
