Niels H. van Dijk

Rate-Induced Solubility and Suppression of the First-Order Phase Transition in Olivine LiFePO4

The impact of ultrahigh (dis)charge rates on the phase transition mechanism in LiFePO4 Li-ion electrodes is revealed by in situ synchrotron diffraction. At high rates the solubility limits in both phases increase dramatically, causing a fraction of the electrode to bypass the first-order phase transition. The small transforming fraction demonstrates that nucleation rates are consequently not limiting the transformation rate. In combination with the small fraction of the electrode that transforms at high rates, this indicates that higher performances may be achieved by further optimizing the ionic/electronic transport in LiFePO4 electrodes.

Taming the First‐Order Transition in Giant Magnetocaloric Materials

Large magnetically driven temperature changes are observed in MnFe(P,Si,B) materials simultaneously with large entropy changes, limited (thermal or magnetic) hysteresis, and good mechanical stability. The partial substitution of B for P in MnFe(P,Si) compounds is found to be an ideal parameter to control the latent heat observed at the Curie point without deteriorating the magnetic properties, which results in promising magnetocaloric properties suitable for magnetic refrigeration.

Direct view on the phase evolution in individual LiFePO4 nanoparticles during Li-ion battery cycling

Phase transitions in Li-ion electrode materials during (dis)charge are decisive for battery performance, limiting high-rate capabilities and playing a crucial role in the cycle life of Li-ion batteries. However, the difficulty to probe the phase nucleation and growth in individual grains is hindering fundamental understanding and progress. Here we use synchrotron microbeam diffraction to disclose the cycling rate-dependent phase transition mechanism within individual particles of LiFePO4, a key Li-ion electrode material. At low (dis)charge rates well-defined nanometer thin plate-shaped domains co-exist and transform much slower and concurrent as compared with the commonly assumed mosaic transformation mechanism. As the (dis)charge rate increases phase boundaries become diffuse speeding up the transformation rates of individual grains. Direct observation of the transformation of individual grains reveals that local current densities significantly differ from what has previously been assumed, giving new insights in the working of Li-ion battery electrodes and their potential improvements.

Three-dimensional magnetic spin-echo small-angle neutron scattering and neutron depolarization: A comparison

The recently developed magnetic spin-echo small-angle neutron scattering (SANS) technique provides unique information about the distance correlation of the local vector magnetization as a function of the spin-echo length within a magnetic material. The technique probes the magnetic correlations on a length scale from 10nm up to 10μm within the bulk of a magnetic material by evaluating the Larmor precession of a polarized neutron beam in a spin-echo setup. The characteristics of the spin-echo SANS technique are discussed and compared to those of the more conventional neutron depolarization technique. Both of these techniques probe the average size of the magnetic inhomogeneities and the local magnetic texture. The magnetic spin-echo SANS technique gives information on the size distribution of these magnetic inhomogeneities perpendicular to the beam and, in principle, independent on the local magnetic induction. This information is not accessible by the neutron depolarization technique that gives the average size parallel to the beam multiplied with the square of the local magnetic induction. The basic possibilities of the magnetic spin-echo SANS technique are demonstrated by experiments on samples with a strong magnetic texture.The recently developed magnetic spin-echo small-angle neutron scattering (SANS) technique provides unique information about the distance correlation of the local vector magnetization as a function of the spin-echo length within a magnetic material. The technique probes the magnetic correlations on a length scale from 10nm up to 10μm within the bulk of a magnetic material by evaluating the Larmor precession of a polarized neutron beam in a spin-echo setup. The characteristics of the spin-echo SANS technique are discussed and compared to those of the more conventional neutron depolarization technique. Both of these techniques probe the average size of the magnetic inhomogeneities and the local magnetic texture. The magnetic spin-echo SANS technique gives information on the size distribution of these magnetic inhomogeneities perpendicular to the beam and, in principle, independent on the local magnetic induction. This information is not accessible by the neutron depolarization technique that gives the averag...

Magnetic detection of small fractions of ferromagnetic martensite within the paramagnetic austenite matrix of TWIP steel

In order to accurately determine a small fraction of a ferromagnetic phase in a paramagnetic matrix, the difference in ferromagnetic and paramagnetic responses to an applied magnetic field has been analyzed. The fraction was examined by field-dependent magnetization measurement at different temperatures as well as by thermo-magnetic measurements at constant magnetic fields. The method was successfully applied for the determination of small martensite fractions in a cold-rolled twinning-induced plasticity austenitic steel at three different thickness reductions.

Operando Nanobeam Diffraction to Follow the Decomposition of Individual Li2O2 Grains in a Nonaqueous Li-O2 Battery.

Intense interest in the Li-O2 battery system over the past 5 years has led to a much better understanding of the various chemical processes involved in the functioning of this battery system. However, detailed decomposition of the nanostructured Li2O2 product, held at least partially responsible for the limited reversibility and poor rate performance, is hard to measure operando under realistic electrochemical conditions. Here, we report operando nanobeam X-ray diffraction experiments that enable monitoring of the decomposition of individual Li2O2 grains in a working Li-O2 battery. Platelet-shaped crystallites with aspect ratios between 2.2 and 5.5 decompose preferentially via the more reactive (001) facets. The slow and concurrent decomposition of individual Li2O2 crystallites indicates that the Li2O2 decomposition rate limits the charge time of these Li-O2 batteries, highlighting the importance of using redox mediators in solution to charge Li-O2 batteries.

Transition Metal Based Magneto Caloric Materials for Energy Efficient Heat Pumps

Magnetic refrigeration near room temperature is considered as an environmentally benign alternative for the current compressor-based cooling technology. Two materials that are based on ferromagnetic transition metal compounds are considered as the most promising candidates to be used in real world applications. Here we discuss the main features of these materials.

Microstructural control of the austenite stability in low-alloyed TRIP steels

We have performed in-situ magnetization and high-energy X-ray diffraction measurements on two aluminum-based TRIP steels from room temperature down to 100 K in order to evaluate amount and stability of the retained austenite for different heat treatment conditions. We have found that the bainitic holding temperature affects the initial fraction of retained austenite at room temperature but does not to influence significantly the rate of transformation upon cooling.

Critical Scaling of the Magnetization and Magnetostriction in the Weak Itinerant Ferromagnet UIr

The weak itinerant ferromagnet UIr is studied by magnetization and magnetostriction. Critical behavior, which surprisingly extends up to several Tesla, is observed at the Curie temperature T C ≃45 K and is analyzed using Arrott and Maxwell relations. Critical exponents are found that do not match with any of the well-known universality classes. The low-temperature magnetization M s ≃0.5 µ B below 3 T rises towards higher fields and converges asymptotically around 50 T with the magnetization at T C . From the magnetostriction and magnetization data, we extract the uniaxial pressure dependences of T C , using a new method presented here, and of M s . These results may serve as a basis for understanding spin fluctuations in anisotropic itinerant ferromagnets.

Contribution to the Understanding of Austenite Stability in a 12Cr-9Ni-4Mo maraging Steel

Maraging steels show an excellent combination of high strength and ductility, which makes them very attractive in a large variety of potential applications. The present work is concerned with the main factors influencing the stability of metastable austenite in such a steel. At subzero temperatures a large variation in the isothermal transformation behaviour of austenite to martensite has been observed. Factors such as the austenite grain size and the interstitial content in solid solution are known to influence austenite stability and, therefore, the martensitic transformation. In this steel, the addition of titanium results in carbonitride precipitation. These precipitates play an indirect but important role in the stability of austenite by means of removing interstitials from the solid solution and by inhibiting an austenite grain growth. The combination of techniques such as X-ray diffraction, magnetisation measurements, three-dimensional neutron depolarisation, and internal friction measurements enables a complete characterisation of the transformation. A step towards understanding the factors responsible for the variation in the behaviour observed is the main contribution of this work.