D.G. de Groot

Synthesis of yttriumtrihydride films for ex-situ measurements

A new method has been developed to synthesize compact yttriumtrihydride by making use of a thin film technique. For electrical measurements yttrium films of typically 500 nm thickness are covered under UHV conditions by a 5 nm thick palladium overlayer which consists of electrically disconnected islands. Loading of these films with hydrogen up to the trihydride phase can then be done ex-situ in a reasonably short time (around 20–40h) by applying gas pressures of about 60 × 105 Pa. For a thicker Pd layer (above 20 nm) this time can be considerably shorter (t ∼ 125 s). The film morphology stays intact during the loading process although the film thickness increases by approximately 11% and the crystal structure changes from h.c.p. to f.c.c. and back to h.c.p. These samples are, therefore, very well suited for an investigation of the remarkable electrical and optical properties of trihydrides, as recently reported by Huiberts et al. (Nature, 380, 1996, 231). In this article we give evidence for the island structure of the palladium overlayer and make a comparison of a number of physical properties of yttrium and its related hydrides as thin films with literature values for the same material in bulk form. These properties include lattice parameters for the different hydride phases, electrical resistivity for yttrium and its dihydride and Hall coefficient for yttrium. The characteristics of the yttriumhydride thin films are very similar to those of bulk material. Furthermore, we performed concentration measurements and resistivity measurements during hydrogen loading. It is shown that the resistivity rises three orders of magnitude when yttrium is loaded up to the trihydride phase at 60 × 105 Pa.

Accumulation of weak bases in relation to intralysosomal pH in cultured human skin fibroblasts

The volume of the lysosomal compartment in cultured human skin fibroblasts was estimated from the distribution between the cells and the medium of tracer amounts of labelled methylamine and chloroquine, which accumulate in the lysosomes, 2,2-dimethyloxazolidine-2,4-dione, which accumulates in the soluble cytoplasmic compartment relative to the lysosomes, and sucrose, which is excluded by the cells. In a foetal fibroblast line, the fractional volume of the lysosomal compartment was 0.044 +/- 0.007 (n = 8). In fibroblasts from a patient with the I-cell disease, the fractional volume was 0.15. The fractional volume of the lysosomal compartment was used to calculate the intralysosomal pH from the accumulation of the weak bases in the cells. The mean value obtained was 5.29 +/- 0.04 (n = 8). In fibroblasts incubated with various concentrations of chloroquine, the fractional volume of the lysosomal compartment and the accumulation of chloroquine in the cells were used to calculate the concentration of chloroquine in the lysosomes. The intralysosomal concentration increased from 3 to 114 mM as the extracellular concentration increased from 1 to 100 microM. Concomitantly, the intralysosomal pH increased from 5.3 in the absence of chloroquine to 5.9 in the presence of 100 microM chloroquine. A similar increase in intralysosomal pH could be calculated in fibroblasts incubated with different concentrations of ammonia.

LASER ABLATION THRESHOLD OF YBA2CU3O6+X

Using projection optics we made a detailed study of the interaction of a spatially uniform 248 nm excimer laser beam and a 99% dense YBa2Cu3O6+x target. Below a threshold fluence of 1 J/cm2 the roughness of the irradiated target increases dramatically due to non‐stoichiometric ablation. The overall target surface composition becomes increasingly Y rich and Cu poor, while the opposite is found for the corresponding ablated thin films. Above the threshold fluence the composition of the ablated target surface is conserved. As a result of the energy homogeneity of the laser beam obtained by means of projection optics, the optimization of the deposition parameters has been improved leading to the reproducible fabrication of flat, stoichiometric YBa2Cu3O7 films with Tc0’s over 91 K.

Pressure dependence of the T/sub c/ of YBa/sub 2/Cu/sub 3/O/sub 7/ up to 170 kbar

The superconducting onset temperature T/sub co/ of single-phased YBa/sub 2/Cu/sub 3/O/sub 9-//sub delta/ (with deltaapprox. =2) measured resistively in a diamond anvil cell is found to increase at a rate dT/sub co//dp = 0.043 K kbar/sup -1/ up to 170 kbar. This is much weaker than for La-Ba-Cu-O for which dT/sub co//dp = 0.64 K kbar/sup -1/. The pressure dependence of the high-T/sub c/ superconductors measured so far cannot be explained within a standard electron-phonon Bardeen-Cooper-Schrieffer theory. Predictions of resonating-valence bonds and bipolaronic theories are discussed.

Effect of time of day on walking capacity and self-reported fatigue in persons with multiple sclerosis: a multi-center trial

Background: Many persons with multiple sclerosis (PwMS) report increased fatigue in the afternoon and evening compared with the morning. It is commonly accepted that physical capacity also decreases as time of day progresses, potentially influencing the outcomes of testing. Objective: The objective of this article was to determine whether self-reported fatigue level and walking capacity are influenced by time of day in PwMS. Methods: A total of 102 PwMS from 8 centers in 5 countries, with a diverse level of ambulatory dysfunction (Expanded Disability Status Scale [EDSS] <6.5), participated. Patients performed walking capacity tests and reported fatigue level at three different time points (morning, noon, afternoon) during 1 day. Walking capacity was measured with the 6-Minute Walk Test (6MWT) and the 10-m walk test performed at usual and fastest speed. Self-reported fatigue was measured by the Rochester Fatigue Diary (RFD). Subgroups with mild (EDSS 1.5–4.0, n = 53) and moderate (EDSS 4.5–6.5, n = 49) ambulatory dysfunction were formed, as changes during the day were hypothesized to depend on disability status. Results: Subgroups had different degree of ambulatory dysfunction (p < 0.001) but reported similar fatigue levels. Although RFD scores were affected by time of day with significant differences between morning and noon/afternoon (p < 0.0001), no changes in walking capacity were found in any subgroup. Additional analyses on subgroups distinguished by diurnal change in self-reported fatigue failed to reveal analogous changes in walking capacity. Conclusions: Testing of walking capacity is unaffected by time of day, despite changes in subjective fatigue.

Relation between IgG antibodies to foods and IgE antibodies to milk, egg, cat, dog and/or mite in a cross-sectional study

Because IgG antibodies to foods can be detected before IgE antibodies to inhalants, increased levels of IgG antibodies to foods might be used as a predictor of IgE‐mediated allergy in initially nonatopic children.

Critical point lowering in thin PdHx films

Pressure-composition isotherms of PdHx films of 100-400 nm thickness have been measured between 300 and 500K at pressures up to 10 bar by means of an oscillating quartz microbalance. For all the films investigated the two-phase region is considerably narrower than that of bulk PdHx. This narrowing of the miscibility gap, together with the observed lowering of the critical temperature, is due to a weakening of the hydrogen-hydrogen interaction which probably originates from the elastic boundary conditions at the film-substrate interface.

Search for new metal-hydrogen systems for energy storage☆

Abstract The heats of formation of the transition metal hydrides A5BHx A2BHx, ABHx, AB2Hx and AB5Hx (A ≡ Sc, Ti, V, Y, Zr, Nb, La, Hf, Ta, Ni, Pd; B ≡ transition metal) are calculated by means of a semiempirical model based on the electronic structure of the host alloy. Out of the 1380 possible compounds A5B, A2B and AB, only 44 are predicted by the model to react with hydrogen with a heat of formation between −12 and −25 kJ. Approximately half of these potential hydrogen storage systems have already been investigated.

Surface Tension and Subsurface Sites of Metallic Nanocrystals Determined from H-Absorption

From pressure-composition isotherm measurements of the absorption of atomic hydrogen in palladium nanocrystals it is possible to determine accurately i) the energy of hydrogen in the bulk and at subsurface sites and ii) the surface tension of the host metal at temperatures well below the melting point of the host metal.

Absorption of hydrogen in Pd-Co and Pd-U alloys☆

Abstract The absorption of hydrogen in the structurally disordered alloys Pd 95 Co 5 , Pd 90 Co 10 and Pd 95 U 5 and in the compound Pd 3 U is studied experimentally by means of pressure-composition isotherms at temperatures between 300 K and 600 K and hydrogen pressures up to 50 bar. In spite of the fact that pure uranium readily absorbs hydrogen, whereas cobalt does not, the absorption of hydrogen is reduced more in the Pd-U alloys than in the Pd-Co alloys. In Pd 3 U no appreciable absorption could be detected at all. This remarkable reduction of hydrogen solubility in Pd-U alloys is attributed to the filling of the palladium 4d band on alloying with uranium.