Henry E. Fischer

Thermal conductivity of thin films: Measurements and understanding

Several techniques are reviewed with which thermal conductivity and phonon scattering can be measured in films of thicknesses ranging from angstroms to millimeters. Recent experimental results are compared critically with previous measurements. It is shown that phonons are very sensitive probes of the structural perfection of the films.

Neutron and x-ray diffraction studies of liquids and glasses

The techniques of neutron diffraction and x-ray diffraction, as applied to structural studies of liquids and glasses, are reviewed. Emphasis is placed on the explanation and discussion of the experimental techniques and data analysis methods, as illustrated by the results of representative experiments. The disordered, isotropic nature of the structure of liquids and glasses leads to special considerations and certain difficulties when neutron and x-ray diffraction techniques are applied, especially when used in combination on the same system. Recent progress in experimental technique, as well as in data analysis and computer simulation, has motivated the writing of this review.

The D20 instrument at the ILL: a versatile high-intensity two-axis neutron diffractometer

D20 is a medium to high resolution two-axis diffractometer capable of producing a neutron flux of 108 s−1 cm−2 at the sample position. The 1536 detection cells of its curved linear position sensitive detector (PSD) cover a continuous 2θ range of 153.6° over a total solid angle of 0.27 sr. This combination of a high incident neutron flux and a large detector solid angle provides D20 with the fastest counting rate, at a given resolution, of any reactor-based neutron diffractometer. Different monochromators and take-off angles, plus optional Soller collimators and secondary slits, permit a wide choice in the Q-space range, wavelength, resolution and flux. A high-resolution configuration offers Δd/d ~ 2 × 10−3. Fast modern counting electronics allow in situ time-resolved experiments at the timescale of a few tens of milliseconds. In addition, a variety of sample environments, including an optional radial oscillating collimator for suppressing parasitic scattering, contribute to a rich scientific programme.

Kinetics of the high- to low-density amorphous water transition

In situ neutron diffraction experiments have been carried out to study the kinetics of the transformation of high-density amorphous (HDA) water into its low-density amorphous state at temperatures 87 K ≤ T ≤ 110 K. It is found that three different stages are comprised in this transformation, namely an annealing process of the high-density matrix followed by a first-order-like transition into a low-density state, which can be further annealed at higher temperatures T ≤ 127 K. The annealing kinetics of the HDA state follows the logarithm of time as found in other systems showing polyamorphism. According to the theory of transformation by nucleation and growth the apparent first-order transition follows an Avrami–Kolmogorov behaviour. An energy barrier ΔE ≈ 33 k Jmol−1 is estimated from the temperature dependence of this transition.

Levitation apparatus for neutron diffraction investigations on high temperature liquids

We describe a new high temperature environment based on aerodynamic levitation and laser heating designed for neutron scattering experiments up to 3000°C. The sample is heated to the desired temperature with three CO2 lasers from different directions in order to obtain a homogeneous temperature distribution. The apparent temperature of the sample is measured with an optical pyrometer, and two video cameras are employed to monitor the sample behavior during heating. The levitation setup is enclosed in a vacuum-tight chamber, enabling a high degree of gas purity and a reproducible sample environment for structural investigations on both oxide and metallic melts. High-quality neutron diffraction data have been obtained on liquid Y3Al5O12 and ZrNi alloy for relatively short counting times (1.5h).

Epitaxial MgO layer for low-resistance and coupling-free magnetic tunnel junctions

Epitaxially grown magnetic tunnel junctions MgO(100)/Fe/MgO/Fe/Co/Pd have been elaborated by molecular beam epitaxy, with insulating layer thickness down to 0.8 nm. The continuity of this layer was checked at different spatial scales by means of morphological (high resolution transmission electronic microscopy), electric (local impedance), and magnetic (magnetoresistance and hysteresis loop) measurements. These junctions show a low resistance (4 kΩ μm2), tunnel magnetoresistance up to 17%, and a very small interlayer magnetic coupling.

Hydrogen bonding in liquid methanol at ambient conditions and at high pressure

Neutron and hard X-ray diffraction measurements on liquid methanol at room temperature are presented at pressures ranging from 1 bar to 9 kbar. Under ambient conditions a complete partial structure factor analysis has been performed. The partial structures are not all in agreement with simulation results. Notably, the data show no sign of a sharp feature in the HO(O/C) partial, implying that the hydrogen between the hydrogen bonded oxygen is less localized than predicted by the standard simulation potentials. The data have been analysed using a continuous random network approach common in the analysis of the structure of tetrahedral network glasses. The pressure-induced change in the scattering intensity is restricted to the momentum transfer range (0 < Q < 2.5 Å−1), i.e., there is no discernible change in sharp real-space structures. Up to 9 kbar the change in scattering intensity scales to a good approximation with the density.

The structure of liquid and supercritical deuterium fluoride from neutron scattering using high-pressure techniques

We present neutron diffraction data for deuterium fluoride at two liquid and four supercritical states in the temperature range of 300–473 K and for pressures up to 320 bar, spanning a factor of 4 in sample density. The intra- and intermolecular parts of the measured structure factors were separated and conclusions about the temperature and density dependence of the intra- and intermolecular structure are deduced.

Interplay between non-bridging oxygen, triclusters, and fivefold Al coordination in low silica content calcium aluminosilicate melts

In the present work the structural properties of low silica content calcium aluminosilicate melts with concentration ratio CaO/Al2O3 = 1 are investigated in the liquid and undercooled states by neutron diffraction experiments and ab initio molecular dynamics simulations. The results show the presence of AlO5 units and triclusters as well as non-bridging oxygen in the fully charge balanced compositions. Moreover, our findings allow us to identify a possible interplay between these structural units. Finally, we discuss the influence of these defective structural units on the properties of tetrahedral network and more particularly their implication on the evolution of the viscosity and the fragility.

A determination of the structure of liquid using neutron diffraction and isotopic substitution

The partial structure factors and pair distribution functions for liquid were measured using the method of neutron scattering and isotopic substitution. The results show many features which are consistent with an ionic melt in which small ions move rapidly through a highly ordered liquid sub-structure in a similar way to the motion of in the fast-ion solid which exists prior to melting. The results are compared with recent molecular dynamics and ab initio molecular dynamics calculations, and the relative merits of the two simulation approaches are discussed in relation to the data.