A novel core material for vacuum thermal insulation, the melamine-formaldehyde (MF) rigid foam was processed from an emulsion of the melamine-formaldehyde resin at temperatures between 130°C and 150°C, using pentane as the blowing agent. The achieved porosity was between 92 and 98 %. Open pore structure with desired mechanical properties was achieved by variations of the initial chemical composition of liquid reactants and controlled foaming and hardening, employing classical heating. Optimization of the synthesis resulted in the base thermal conductivity equal to only 0.006 W m-1 K-1 and an extremely low outgassing rate. The long-term pressure-rise measurements indicate that these MF rigid foams could be the first organic candidates applied as the core material in Vacuum Insulating Panels (VIPs) whose performance may be comparable to selected inorganic core materials. Their further advantage compared to conventional organic foams is their stability, as they can withstand a temperature in excess of 200 °C, self-extinguishability.
COBISS.SI-ID: 27183911
Quantification of hydrogen fraction in the gas mixture with inert gases kept in a small enclosure of the gas surge arrester (GSA) is a challenging task. Hydrogen greatly influences device properties, but as an omnipresent gas it represents also the background of any mass spectrometer. Hydrogen fraction in a particular GSA was quantified after its puncture in an evacuated batch inletand subsequent introduction to a pumped chamber housing a quadrupole massspectrometer (QMS). Its calibration was performed by an innovative in-situcalibration procedure which should yield high accuracy. In the first stage, a pure gas (Ar, Ne, H2) contained in a calibrated volume was set by a leak valve to flow into the UHV system. The pressure change reading of the capacitance manometer over time gives the flow rate which is directly correlated to the ion current of a specific mass peak in the span of 3 orders of magnitude. The extracted calibration curves of the QMS for each gas speciesare applied in the second stage of the calibration when known gas mixtures are prepared in the calibrated volume to verify the gas composition determination procedure based on the fractionation. Such procedure is revealedfairly accurate at high (above -6 X 10-5 mbar L/s) flow rates, howevera significant error appeared at lower flow rates. Possible explanationsfor erroneous hydrogen determination at very low fluxes by the QMSin the mixture with argon are presented.
COBISS.SI-ID: 1977191
Nitrogen doped, hydrogen terminated diamond films have shown a work function of less than 1.5 eV and thermionic electron emission (TE) has been detected at temperatures less than 500 °C. However, ambient exposure or extended operation leads to a deterioration of the emission properties. In this study thermionic electron emission has been evaluated for as-received surfaces and for surfaces after 18 months of ambient exposure. The initial TE current density of the freshly deposited diamond film was ~5x10-5A/cm2 at 500 °C. In contrast, the initial TE current density of a film aged for 18 months was ~1.8x10-9A/cm2 at 500 °C. The decreased emission current density is presumed to be a consequence of oxidation, surface adsorption of contaminants and hydrogen depletion from the surface layer. In situ reactivation of the aged film surface was achieved by introducing hydrogen at a pressure of 1.3x10-4 mbar and using a hot filament of a nearby ionization gauge to generate atomic and/or excited molecular hydrogen. After two hours exposure with the sample at 500 °C, the surface exhibited a stable emission current density of ~ 2.3x10-6 A/cm2 (an increase by a factor of ~1300). To elucidate the reactivation process thermionic electron energy distribution (TEED) and XPS core level spectra were measured during in situ hydrogen exposure at 5x10-8 mbar. During the isothermal exposure it was determined that atomic or excited hydrogen resulted in a much greater increase of the TE in comparison to exposure to molecular hydrogen. During exposure at 400 °C the surface oxygen was substantially reduced, the TEED cut-off energy, which indicates the effective work function, decreased by ~200 meV, and the TE intensity increased by a factor of ~ 100. The increase in thermionic emission with hydrogen was ascribed to the reactivation of the surface through the formation of a uniform surface dipole layer and a reduction of the surface work function.
COBISS.SI-ID: 28087847
The hydrogen outgassing rate from three equivalent UHV chambers made of 0.5-mm-thick AISI 204 stainless steel was measured in situ over several 10-h cycles at 150 °C, 200 °C and 250 °C. After each cycle, the specific outgassing rate at room temperature was determined using a spinning rotor gauge. A parallel analysis based on thermal desorption spectroscopy, extraction melting and numerical simulation of the recombination limited process reveals a more consistent picture of the outgassing process compared with models given in old vacuum textbooks. At the applied moderate temperatures, the majority of the diffusible hydrogen is released within less than 100 h. Consequently, the specific outgassing rate at room temperature was reduced to the level of q (1 × 10−13 mbar L/cm2s. At temperatures above 400 °C, the residual hydrogen is activated and is depleted very slowly. It seems that it represents an inner source of hydrogen that is manifested by a stable outgassing rate at moderate temperatures and on the long-term scale at room temperature.
COBISS.SI-ID: 27971879
We investigate the role of the substrate morphology in the dewetting of ultrathin chromium films irradiated with 30 keV Ga ions. Silicon surfaces with different roughness were used as substrates for the films. The results of the irradiation experiments and of related simulations indicate that the chromium films can undergo a dewetting-like process through the two standard channels that show up for liquids, namely the spinodal channel, and the dewetting by heterogeneous nucleation. The two processes are competitive, and the prevailing one can be predicted and selected according to the characteristics of the substrate.
COBISS.SI-ID: 1060010