Richard Dunlap

A Mossbauer effect investigation of the Li-Sn system

Abstract A thorough Mossbauer effect investigation of all the phases in the Li–Sn binary phase diagram has been made. The compounds studied were verified to have the same crystal structures as those reported in the literature. The center shifts (relative to CaSnO 3 ), quadrupole splittings, number of Sn sites and their abundance have been determined for each of the phases by fitting model spectra to the observed data. The observed spectra show mean center shifts near 2.4 mm/s for the Sn-rich phases (Sn, Li 2 Sn 5 and LiSn) and mean center shifts between 2.1 and 1.8 mm/s for the Li-rich phases (Li 7 Sn 3 , Li 5 Sn 2 , Li 13 Sn 5 , Li 7 Sn 2 and Li 22 Sn 5 ). These results are important for the interpretation of Mossbauer effect measurements on high capacity Sn-based electrode materials for Li-ion batteries.

Nanostructured materials for lithium-ion batteries: Surface conductivity vs. bulk ion/electron transport

Lithium metal phosphates are amongst the most promising cathode materials for high capacity lithium-ion batteries. Owing to their inherently low electronic conductivity, it is essential to optimize their properties to minimize defect concentration and crystallite size (down to the submicron level), control morphology, and to decorate the crystallite surfaces with conductive nanostructures that act as conduits to deliver electrons to the bulk lattice. Here, we discuss factors relating to doping and defects in olivine phosphates LiMPO4 (M = Fe, Mn, Co, Ni) and describe methods by which in situ nanophase composites with conductivities ranging from 10(-4)-10(-2) S cm(-1) can be prepared. These utilize surface reactivity to produce intergranular nitrides, phosphides, and/or phosphocarbides at temperatures as low as 600 degrees C that maximize the accessibility of the bulk for Li de/insertion. Surface modification can only address the transport problem in part, however. A key issue in these materials is also to unravel the factors governing ion and electron transport within the lattice. Lithium de/insertion in the phosphates is accompanied by two-phase transitions owing to poor solubility of the single phase compositions, where low mobility of the phase boundary limits the rate characteristics. Here we discuss concerted mobility of the charge carriers. Using Mössbauer spectroscopy to pinpoint the temperature at which the solid solution forms, we directly probe small polaron hopping in the solid solution Li(x)FePO4 phases formed at elevated temperature, and give evidence for a strong correlation between electron and lithium delocalization events that suggests they are coupled.

An investigation of mechanically alloyed Fe–Al

Abstract An X-ray diffraction and 57 Fe Mossbauer effect spectra study of mechanically alloyed Fe100−xAlx is presented. Alloys with 0≤x≤60 were prepared from elemental powders in a high energy ball mill. Alloys of the composition Fe50Al50 were also studied in detail as a function of milling time in order to better understand the phase formation in these alloys. X-ray measurements showed all fully milled samples to be of the bcc structure. Up to x=40, Mossbauer effect spectra showed the alloys to be ferromagnetic with a decreasing mean hyperfine field as a function of x. For x>40 no magnetic splitting was observed indicating that the alloys are paramagnetic at room temperature. Hyperfine field distributions of the ferromagnetic alloys can be interpreted in terms of Fe neighbor environments as a function of Al content. As a function of milling time, the composition of the Fe50Al50 alloy may be described in terms of changing proportions of three different phases; a ferromagnetic bcc phase (FeAly), a paramagnetic fcc phase (AlFey) and a paramagnetic bcc phase (FeAl). The relative proportions, compositions and microstructures of these phases may be understood from the combined X-ray diffraction and Mossbauer effect measurements.

Al-M ( M = Cr , Fe , Mn , Ni ) Thin-Film Negative Electrode Materials

Comprehensive investigations of thin-film Al-M (M = Cr, Fe, Mn, Ni) negative electrode material libraries were performed using combinatorial and high-throughput techniques. Mossbauer effect spectroscopy and X-ray diffraction were used to characterize library structure. The electrochemical performance of over 200 compositions of Al x M 1-x (0.75 < x < 1) was determined at both room and elevated temperature. All Al x M 1-x alloys are completely inactive above 15 atom % M. Thermodynamic calculations based on the macroscopic atom model of de Boer et al. predict compositions with at least 33-65 atom % M to be inactive. A phenomenological model to describe the observed capacity as a function of transition metal content and cycling temperature is presented.

Mössbauer effect studies of Fe100−xGax films prepared by combinatorial methods

Magnetostrictive Fe100−xGax alloys over the composition range 0 20, the Mossbauer effect results suggest the presence of short range D03 type order. This persists up to the maximum Ga content studied here (x = 36), although there is no evidence to support the formation of long range D03 structural order in any of the films prepared in this study.

Combinatorial Study of the Sn–Cu–C System for Li-Ion Battery Negative Electrode Materials

Thin-film combinatorial sputter-deposited Sn-Cu-C alloys for negative electrodes of Li-ion batteries were characterized using electron microprobe (energy-dispersive spectroscopy), X-ray diffraction, electrochemical methods, and 119 Sn Mossbauer effect spectroscopy. Combinatorial libraries with nominal compositions Sn x Cu y C 1―x―y , with x = 0.49, x = 0.39, and x = 0.25 (0 < y < 1 - x) and Sn x Cu y C 1―x―y with y/x = 6/5 (0 < 1 ― x ― y < 0.45) were investigated. The addition of carbon was responsible for a decrease in grain size and an increase in specific capacity, which approached the calculated value (based on 4.4Li/Sn + 0.5Li/C). Mossbauer effect spectra were compared with differential capacity (dQ/dV vs V) and it was determined that the sample with the best capacity retention most closely resembled nanostructured Cu 6 Sn 5 grains embedded in a C matrix, yielding over 90% capacity retention after 30 cycles. However, changes in dQ/dV vs V suggested that regions of Cu 6 Sn 5 were steadily growing or aggregating as Li was cycled, which may be a limiting factor in later cycles.

Microstructure and low temperature magnetism of Fe-Ni invar alloys

Abstract We report measurements of the low field (≈ 10 G) near zero field quenched (earths field) SQUID magnetization, MZFQ, measured on warming in the interval 4.2–250 K for 8 as splat quenched and annealed FeNi alloy samples with 68–35 at % Fe. The samples were characterized by Mossbauer spectroscopy and vibrating sample magnetometry. The as quenched samples showed the predicted MZFQ behaviour for “strong” ferromagnetic domain wall pinning, which we attribute to quenched-in crystallographic defects. Annealing effectively removed the latter defects, however, in most samples it concomitantly introduced α/γ′-phase precipitation precursors. The 68 at % Fe sample in which precursors were not formed (due to the high temperature of its single phase boundary) showed reentrant spin glass (RSG) behaviour, as seen previously in virgin state samples. The RSG behaviour is believed to arise from pinning due to intrinsic domain wall width scale compositional variations — combined with demagnetization effects and/or normal thermal randomization. No evidence is seen for a true RSG transition or phase occurring in FeNi alloys.

Mössbauer effect and X-ray diffraction investigation of Si-Fe thin films

An X-ray diffraction and 57Fe Mössbauer spectroscopy investigation of Si100– x Fe x (0 < x < 80) thin films prepared by combinatorial sputtering methods is reported. Resulting Mössbauer spectra were fit to Voigt-based distributions of quadrupole doublets for paramagnetic spectral components and Zeeman split sextets for ferromagnetic spectral components. In conjunction with the X-ray measurements, these results show that the Si-rich films are a mixture of dilute Fe in amorphous Si and an approximately equiatomic amorphous SiFe phase. Fe-rich films show the presence of a ferromagnetically ordered phase. For x < 73, this ferromagnetic phase is amorphous or nanostructured and for x ≥ 73, the phase is shown to be a crystalline bcc phase. Results are discussed in terms of short-range structural ordering in these alloys.

Development of ⟨1 0 0⟩ crystallographic texture in magnetostrictive Fe–Ga wires using a modified Taylor wire method

Magnetostrictive Fe?Ga wires 1?3?mm in diameter have been prepared using an innovative cost-effective approach?based on the Taylor wire method?that couples the advantages of high-temperature directional solidification and selective grain growth. Strict control of drawing processes has been shown to enable the introduction of desirable texture that is critical for achieving large magnetostriction in these polycrystalline Fe?Ga alloys. The procedure for fabricating highly textured 1?0?0 magnetostrictive wires is discussed and the wires are evaluated in terms of microstructure and crystallographic texture. Magnetostriction measurements, in the absence of pre-stress and stress-annealing treatments, indicated a maximum magnetostriction of ~170?ppm in a saturation field less than 60?mT. A mechanism for texture evolution is proposed. It is speculated that the resultant 1?0?0 texture of the Fe?Ga wires is due to directional solidification and abnormal grain growth resulting from surface effects. The unique properties of wires made with the Taylor-based approach coupled with the low fabrication cost make this an attractive approach for the production of Fe?Ga wire with a specific crystallographic texture.

Influence of Processing on Crystallographic and Magnetic Structure of Fe-Ga

Methods for fabrication of Fe-Ga films and melt spun ribbons were investigated and their influence on microstructure and texture were explored. The materials were characterized by electron microprobe analysis, Mossbauer spectroscopy, and X-ray diffraction. Characterization of the films showed that composition varied linearly with position on the film and that all compositions have the disordered bcc structure. Mossbauer effect spectra show no evidence for the presence of long-range ordering, that may diminish the magnitude of the magnetostriction.