M. Jäckel

Thermal conductivity of modified epoxy resins with different cross-link densities at low temperatures

Abstract The thermal conductivity of modified epoxy resins was investigated from 0.5 to 80 K. Two series of samples with different cross-link densities were used. The results are discussed.

Thermal properties of polymer/particle composites at low temperatures

Abstract With different models, the thermal conductivity of composite materials is calculated on the basis of the thermal conductivity of the matrix and filler, of the volume fraction and shape of the fillers and with consideration of the thermal boundary resistance in the temperature range below 20 K. Measurements of the thermal conductivity and specific heat of epoxy resins with different fillers (needle-shape Ag, HTSC powders) in the temperature range 2–80 K are presented. Comparison of the measured and calculated thermal conductivity of these composites shows that above 20 K, the thermal conductivity is determined to a high degree, by the shape of the fillers.

Boundary layer induced modification of thermal and mechanical properties of epoxy resin composites

Abstract When predicting and altering the properties of composites, one has to consider not only the peculiarities of the components themselves but also the boundary layer between the filler and the polymer matrix. The aim of this paper is to study the influence of the boundary layer on the low temperature thermal conductivity and specific heat of an epoxy resin/particle composite. Measurements of the glass transition temperature and the hardness are interpreted, as obtained from the filler surface, and electron micrographs of the boundary layer are presented.

Low-temperature thermal conductivity of amorphous Polycarbonat under high pressure*

We measured the thermal conductivity λ of Polycarbonat (PC) in the temperature range from 3 K to 150 K under application of hydrostatic pressure up to 1.3 GPa. A strong influence of pressure on λ was observed over the entire temperature range. An additional measurement of the pressure dependence of density showed that the density of PC increases appreciably with pressure. The thermal conductivity data were thus corrected considering the changes of density. The corrected data show an increase of λ with pressure only in the plateau region of thermal conductivity. Similar results were measured for vitreous silica [1] and epoxy resin [2], where the data of vitreous silica show an opposite behaviour of λ with increasing pressure.

Low-temperature thermal properties of amorphous polycarbonat

The thermal conductivity λ, the specific heat c and the thermal expansion α of isotropic Polycarbonat (PC) and of PC after stretching were measured in the temperature ranges between 0.1 K and 80 K (λ, c) and between 4 K and 300 K (α). The measurements show the typical low-temperature behavior of amorphous solids. Below 20 K the results can be explained in the framework of the soft potential model. Above 20 K with increasing temperature the thermal conductivity and the thermal expansion show between 30 K and 50 K a distinct decrease in slope of the temperature dependence. PC, stretched at 438 K with a load of 2 kN, shows a high anisotropy in these two thermal properties.

Thermal and Dielectric Properties of Epoxy Resin at Low Temperatures after Irradiation and H2 Permeation of Fibre Composites at Room Temperature

Measurements of thermal conductivity, specific heat, and dielectric properties of an epoxy resin after electron, gamma, fast-neutron, and thermal-neutron irradiation are presented. The experimental results show the influence of the different types of radiation on the typical amorphous behaviour of the thermal properties. The H2-permeation rate for two types of carbon-fibre-reinforced plastics are presented.

Adsorption of hydrogen, neon, and helium on 5A molecular sieve at 20.4 K and 4.2 K in the low pressure range

Abstract The quality of the heat contact existing between adsorbent and cooled surface has a decisive influence on pumping speed. By gluing the adsorbent onto the surface by means of a water glass-talcum mixture, the pumping speed, as compared to that of the loose layers, could be increased by a factor of about 200. The pumping speed is not considerably reduced when several load cycles are made with the adsorbent being heated to room temperature only between cycles.

Adsorption of hydrogen by condensed carbon dioxide at 20 K

Abstract The specific adsorption capacity is independent of the thickness up to layer thicknesses of 15 μm, and for thicker layers it decreases. At a given specific load the highest pumping speed is obtained with the thinnest layers. With increasing pressure pumping speed rises at equal absolute load, whereas with equal percentage load a drop is observed. It is possible to regenerate the adsorbent by condensing new layers of carbon dioxide on the charged ones.

Low-temperature thermal conductivity of diolmodified epoxies

SummaryThe thermal conductivity at low temperatures (between 0.5 K and 100 K) was measured for diolmodified epoxies. Diglycidyl ether of bisphenol A (DGEBA) was modified for this purpose by aliphatic diols with the structure HO-(-CH2-)n-OH in the presence of catalyst (either N,N-dimethyl benzylamine or magnesium perchlorate). Sample series with diols of n=4,8 and 12 were synthesized and measured. The results at T<20K shows a clear dependence of the thermal conductivity values on the chain length of the diols. The increasing amount of diol in the epoxies cause a larger change on these values in the same temperature range.

Influence of high pressure on the specific heat of amorphous polymers at low temperatures

The specific heat cp of amorphous polycarbonate and amorphous polystyrene has been measured at pressures up to p = 0.7 GPa in the temperature range 0.2K <T<300K. The specific heat shows a visible dependence on pressure, specifically in the temperature range at and below the typical maximum of c p /T 3 . With increased pressure, the temperature T max of the cp-maximum and the temperature T min of the c p -minimum are slightly shifted to higher values. This is an indication that the energy spectrum of the low-energy vibrations shifts to higher energies with increased pressure.