Luise Theil Kuhn

Stability effects of AunXm+ (X = Cu, Al, Y, In) clusters

Bimetallic AunXm clusters (X=Cu, Al, Y, In) have been produced by a dual-target dual-laser vaporization source. Following multiphoton absorption the stability patterns resulting from fragmentation are investigated by time-of-flight mass abundance spectrometry. AunCum+ clusters exhibit the same electronic shell effects as Aun+. Different abundance patterns are observed for AunAl1+ compared to AunY1+ or AunIn1+. The patterns are related to the magic numbers of the electronic shell model for clusters. The differences between the bimetallic clusters are interpreted in terms of different cluster geometries dependent on the nature of the dopant atoms.

Nanoscale Chemical Analysis and Imaging of Solid Oxide Cells

The performance of solid oxide cells (SOCs) is highly dependent on triple phase boundaries (TPBs). Therefore, detailed TPB characterization is crucial for their further development. We demonstrate that it is possible to prepare a ∼50 nm thick transmission electron microscopy (TEM) lamella of the interface between the dense ceramic electrolyte and the porous metallic/ceramic hydrogen electrode of an SOC using focused ion beam milling. We show combined TEM/scanning TEM/energy-dispersive spectroscopy investigations of the nanostructure at the TPBs in a high-performance SOC. The chemical composition of nanoscale impurity phases at the TPBs has been obtained with a few nanometers lateral resolution.

Transmission Electron Microscopy Specimen Preparation Method for Multiphase Porous Functional Ceramics

An optimum method is proposed to prepare thin foil transmission electron microscopy (TEM) lamellae of multiphase porous functional ceramics: prefilling the pore space of these materials with an epoxy resin prior to focused ion beam milling. Several advantages of epoxy impregnation are demonstrated by successful preparation of TEM specimens that maintain the structural integrity of the entire lamella. Feasibility of the TEM alignment procedure is demonstrated, and ideal TEM analyses are illustrated on solid oxide fuel cell and solid oxide electrolysis cell materials. Some potential drawbacks of the TEM specimen preparation method are listed for other samples.

Strong Metal–Support Interaction: Growth of Individual Carbon Nanofibers from Amorphous Carbon Interacting with an Electron Beam

Great interest has been stimulated in probing the growth behavior and mechanisms of carbon nanofibers (CNFs), as CNFs are known to have electronic, mechanical, chemical, and optical properties that can be exploited for new applications. In principle, high-temperature, decomposition of carbon-containing gases over catalytic metals (for instance, Co, Fe, and Ni), or coupling of these two factors, is indispensible for growth of CNFs. The synthesis methods of CNFs include arcdischarge, laser ablation, and chemical vapor deposition. To grow CNFs from a solid carbon source, the provided energy is usually utilized to enhance the activity of the metal catalysts involved. Amorphous carbon, dispersed with catalytic nickel particles in several tens of nanometers, can be transformed to CNFs while interacting with an electron beam. For a solid carbon system, however, it is still an open question to establish if nickel catalysts smaller than tens of nanometers can trigger the formation of CNFs, when exposed to an electron irradiation without external heating. Moreover, approaching a limit, what will occur when the catalyst particles are exchanged with atomic-scale nickel? To answer this question, a confined space must be established to effectively confining a mixture of solid carbon and catalyst nanoparticles. The porosity of a porous ceramic is an ideal platform for studies of this irradiation effect. Herein we report that CNFs grew from amorphous carbon, which was physically mixed with metastable nanoparticles of fluorite-type Ce0.8Gd0.2O1.9 and 10 wt. % Ni ions (CGO/Ni), in the porous anode of a solid oxide fuel cell (SOFC), stimulated by an electron beam in a 300 kV transmission electron microscope (TEM). The underlying mechanism is ascribed to the irradiation induced instability of the nanoparticles. The strong metal–support interaction (SMSI) effect is activated between the exposed nickel ions and amorphous carbon. Thus the rearrangement of carbon atoms occurring in the mixtures leads to the growth of CNFs without any gaseous carbon source and external heating. The result provides new insight into understanding the growth behavior of CNFs. The sample is a porous anode of an SOFC: the backbone is a composite of Zr0.84Y0.16O1.92 and Sr0.94Ti0.9Nb0.1O3 (STN/YSZ) infiltrated with CGO/Ni electro-catalytic nanoparticles (see the Experimental Section). As illustrated by “Nano@pore” in Figure 1 a, the CGO/Ni nanoparticles are well distributed in the pore sites around large STN/YSZ grains in the anode. As the pore sites were already impregnated with epoxy, the nanoparticles are physically mixed with amorphous carbon (cured epoxy) at nanoscale. The infiltrated nanophases assemble with amorphous carbon forming continuous composites of carbonCGO/Ni around STN/YSZ. As shown in the inset of Figure 1 a, illustrated by the continuous electron diffraction rings, the four main crystal planes of a fluorite structure (CaF2 type) similar to the CGO phase, (111), (2 0 0), (2 2 0) and (3 11) were identified for the CGO/Ni nanoparticles. Considering that CGO/Ni keeps the fluorite structure of CGO phase (space group Fm3̄m, a = b = c = 5.43 , a= b=g= 908), Ni has been well incorporated into the CGO lattice (although a minority of Ni ions may stay on the surface of CGO nanoparticles). This can occur if, a) Ni + substitutes Ce /Gd + in the fluorite structure or b) Ni + is at the interstitial sites in the fluorite structure. Incorporation of Ni can be possible considering the large difference of ionic radius between Ni + (0.69 ) and Ce/Gd (0.97/ 0.938 ). 16] In fact, it has previously been reported for CGO that the fluorite structure can be preserved even when the atomic ratio of Gd/(Ce+Gd) increases from 20 % (Ce0.8Gd0.2O1.9) to 30 % (Ce0.7Gd0.3O1.85) with intermediation of more oxygen vacancies, 18] indicating the possibility of incorporating heterogeneous ions into the CGO phase. With the occurrence of the substitution, besides CGO, the fluorite-type Ce(Ni)O2 [19] was observed to also form. However, it is not possible to distinguish these two phases using selected area electron diffraction pattern (SAED) analysis or high-resolution TEM (HRTEM) imaging in that they not only keep the identical fluorite structure but also maintain the similar lattice parameters. So far, we can’t distinguish whether we observe case a) or b) or a mixture of them. The HRTEM image (Figure 1 b) shows the CGO/Ni nanoFigure 1. a) TEM image of the anode of the SOFC. The inset in (a) is the corresponding SAED analysis of the nanoparticle region. b) HRTEM image of the nanoparticle region.

Effect of stress on NiO reduction in solid oxide fuel cells: a new application of energy-resolved neutron imaging

Recently, two new phenomena linking stress field and reduction rates in anode-supported solid oxide fuel cells (SOFCs) have been demonstrated, so-called accelerated creep during reduction and reduction rate enhancement and nucleation due to stress (Frandsen et al., 2014). These complex phenomena are difficult to study and it is demonstrated here that energy-resolved neutron imaging is a feasible technique for combined mechanics-chemical composition studies of SOFC components, including commercially produced ones. Cermet anode supports, which prior to the measurements were reduced under varying conditions such as different temperatures, various times and different values of applied stress, have been measured. Thus, samples with different contents (and gradients) of Ni and NiO phases were investigated. The first Bragg edge transmission neutron measurements applied for the studies of the reduction progress in these samples were performed at two neutron beamline facilities (ISIS in the UK, Helmholtz Zentrum Berlin in Germany). The obtained results demonstrate the possibility to image and distinguish NiO and Ni phases within SOFC anode supports by energy-resolved neutron imaging and the potential of the neutron imaging method for in situ studies of reduction processes. (Less)

In situ diagnostics of the crystal-growth process through neutron imaging: application to scintillators

The unique possibilities enabled by neutron imaging for in situ remote diagnostics of microstructural characteristics during crystal growth are demonstrated, even when the materials and surrounding structures are opaque to other more conventional interrogation techniques. Neutron radiography is implemented to image remotely the uniformity of elemental distribution (e.g. dopant concentration) during crystal growth, the location of the liquid/solid interface and the presence of macroscopic crystal defects (e.g. cracks), all with a temporal resolution of 5–7 s.

Flexible sample environment for high resolution neutron imaging at high temperatures in controlled atmosphere

High material penetration by neutrons allows for experiments using sophisticated sample environments providing complex conditions. Thus, neutron imaging holds potential for performing in situ nondestructive measurements on large samples or even full technological systems, which are not possible with any other technique. This paper presents a new sample environment for in situ high resolution neutron imaging experiments at temperatures from room temperature up to 1100 °C and/or using controllable flow of reactive atmospheres. The design also offers the possibility to directly combine imaging with diffraction measurements. Design, special features, and specification of the furnace are described. In addition, examples of experiments successfully performed at various neutron facilities with the furnace, as well as examples of possible applications are presented. This covers a broad field of research from fundamental to technological investigations of various types of materials and components.

Three Dimensional Polarimetric Neutron Tomography of Magnetic Fields

Through the use of Time-of-Flight Three Dimensional Polarimetric Neutron Tomography (ToF 3DPNT) we have for the first time successfully demonstrated a technique capable of measuring and reconstructing three dimensional magnetic field strengths and directions unobtrusively and non-destructively with the potential to probe the interior of bulk samples which is not amenable otherwise. Using a pioneering polarimetric set-up for ToF neutron instrumentation in combination with a newly developed tailored reconstruction algorithm, the magnetic field generated by a current carrying solenoid has been measured and reconstructed, thereby providing the proof-of-principle of a technique able to reveal hitherto unobtainable information on the magnetic fields in the bulk of materials and devices, due to a high degree of penetration into many materials, including metals, and the sensitivity of neutron polarisation to magnetic fields. The technique puts the potential of the ToF time structure of pulsed neutron sources to full use in order to optimise the recorded information quality and reduce measurement time.

Effect of heat treatment of pure and carbon-polluted rhodium samples on the low-temperature resistivity

We present a systematic investigation of conditions for heat treatment of Rh with the aim of increasing the residual resistivity ratio (RRR). The maximal value of RRR for a 25 μm thick foil was found to be 1050 and the optimal treatment conditions were high temperatures, above 1400°C, and a low pressure of pure oxygen, around 1 μbar. Another batch of foils, containing less magnetic impurities, showed an RRR of only 600. A 0.4 mm thick single crystal was heat treated to an RRR value of 740. Our findings are discussed in the light of a model with magnetic and non-magnetic impurities in Rh, where the latter is found to have an important contribution for this unusual metal. Especially carbon impurities were found to be quite detrimental for the resistivity, and the recovery of the RRR after a carbon contamination is extremely slow in subsequent heat treatments.

Size effect studies in catalysis: a simple surfactant-free synthesis of sub 3 nm Pd nanocatalysts supported on carbon

Supported Pd nanoparticles are prepared under ambient conditions via a surfactant-free synthesis. Pd(NO3)2 is reduced in the presence of a carbon support in alkaline methanol to obtain sub 3 nm nanoparticles. The preparation method is relevant to the study of size effects in catalytic reactions like ethanol electro-oxidation.