Melamine-formaldehyde fibers were synthesized from a meltable pre-polymer of etherified melamine-formaldehyde in a form of a low density fleece, subsequently thermally cured in a conveyor belt oven at temperatures of up to 200 °C and post-heated at 260 °C. High thermal stability and small fiber diameter below 5 μm made it a serious candidate as a novel core material for vacuum thermal insulation panels. Two most crucial core properties, thermal conductivity and outgassing rate, were investigated in thin-walled stainless steel envelopes, enabling thermal processing combined with a pump-out procedure. A base thermal conductivity of ∼2.3 mW m−1 K−1 was achieved with randomly oriented fibers at a density of ∼250 kg m−3. The long-term pressure-rise measurements revealed extremely low outgassing rates, q ∼ 10−15 mbar L s−1 cm−2. Additional measurements of thermal conductivity in a wide pressure range from 10−3 mbar to the atmosphere indicate that these melamine-formaldehyde fibers could be the first organic candidates applied as the core material in vacuum insulating panels with an adequate service lifetime. Their performance is comparable to selected inorganic core materials like glass fibers.
COBISS.SI-ID: 28332327
Miniaturization of modern sealed vacuum devices and higher demands for their stable operation require accurate determination of the gas composition in the early stage of their operation as well as after a long operational period. Among a few highly sensitive gas methods, quadrupole mass spectrometry seems to be the most appropriate one for this task as gas amounts are well below 1•10−4 mbar L. In this paper, a new approach, how to prepare a quadrupole mass spectrometer for routine quantitative analysis of small gas amounts, is described. In the first stage, it was calibrated by an innovative in-situ procedure using three different gases: nitrogen, argon and neon. Each gas was admitted into a chamber with a precisely determined volume equipped by a capacitance manometer. By opening the variable leak valve, ion current versus flow rate dependence was determined over three orders of magnitude. Non-linear response at very low flow rates was detected. In the next stage, gas quantities from 3•10−5 mbar L to 6•10−7 mbar L of pure gas were admitted, which proved that by numerical correction of non-linearity, the achieved accuracy of gas quantity determination could be noticeably improved. Rather poor results were on the contrary obtained when synthetic gas mixtures were analysed. Additional calibrations with gas mixtures revealed a strong interference provoked by argon. It enhanced the instrument's sensitivity for nitrogen for a factor of up to 2.3, depending on argon fraction. Considering this effect, further corrections led to a substantially improved accuracy of the nitrogen fraction determination in small gas amount mixtures. In general, quantification of very small gas amounts needs a careful analysis of all contributions influencing the accuracy, as the number of data points is limited and mostly recorded in the non-linear mode of the instrument's operation.
COBISS.SI-ID: 28626215
The Se adsorption at different coverages on DO3 FeSiAl(110) surface is studied using density functional theory (DFT). Se adsorption is favorable in almost all surface high-symmetry sites, except for the bridge site formed by Fe-Si atoms. The most stable is a hollow site formed by four Fe atoms with adsorption energy of -5.30 eV. When the coverages increase, the energies decrease in the case of hollow sites. The surface present a reconstruction after Se adsorption, being the most important at 1/2 ML. The local magnetic moment for Fe atoms increase for the type A (all nearst neighbours (nn) are Fe atoms) and decrease for the type B (nn are Fe, Si and Al atoms). The most affected metal orbitals are Fe 4s and 4p. In the case of the hollow site the surface Fe-Fe bond is weakened after Se adsorption. A Fe-Se bond is developed at all coverages in both sites being the most important on top (dFe-Se = 2.23 Å, OP: 0.774 at 1/4 ML). The first and second layer Fe-Fe bond increase at 1/4 ML and decrease at 1/2 and 1 ML. Small Se-Se bonding interaction appear at 1/2 ML and increase noticeable for 1 ML. For the top site, the Se-Se bond appears at all coverage. The Fe-Fe surface bonds also decrease its strength with respect to the clean surface at all coverage. The first and second layer Fe-Fe bond increase at all coverage.
COBISS.SI-ID: 1150890
The spinning rotor gauge (SRG) is a common transfer standard in key comparisons (KCs) and other intercomparisons for pressures in the range of 1.0 x 10-4 Pa to 1.0 Pa. To make absolute pressure measurements using a SRG, a calibration factor, known as the accommodation coefficient, must be determined. Comparisons which utilize SRGs require each participant to determine the accommodation coefficient. The accommodation coefficient of an SRG is known to have excellent long-term stability ((0.1% over 1 year; k = 1) in a laboratory environment where the rotor remains undisturbed and attached to a vacuum standard, but the long-term stability of SRGs used in comparisons is often worse than what is observed in the participants own laboratory. Recently, the Bureau International des Poids et Mesures Consultative Committee for Mass and Related Quantities Working Group on Low Pressures held a workshop to discuss the stability of the accommodation coefficient in inter-laboratory comparisons. Here we summarize the data presented during the workshop and the ensuing discussions and give a list of recommended practices derived from the workshop.
COBISS.SI-ID: 1118634
A new outgassing rate facility applying the difference method was established at KIT. This system was validated within the framework of Joint Research Project IND 12 of the European metrology Research programme by several outgassing reference probes. These served as transfer standards for comparison with three other outgassing rate systems available in the project. Two of the probes were newly developed reference devices and exhibit a known, constant outgassing rate of water. Further two probes were also newly developed outgassing reference devices which emit mixtures of gases. Additionally another probe was an elastomer which was loaded with a known gas mixture. The last probe was a commercially available plastic material whose outgassing shows a large spectrum of gases. The other outgassing rate facilities applied the in-situ calibration of quadrupole mass spectrometers by a primary method, the continuous expansion method (Physikalisch-Technische Bundesanstalt) and the known gas quantity method (Institute of Metals and Technology) or the throughput method (VACOM company). The comparison of the results shows a satisfactory agreement between the measurements.
COBISS.SI-ID: 1194666