D. H. Ryan

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.

Magnetic nanostructured composites using alginates of different M/G ratios as polymeric matrix

Alginate extracted from Sargassum fluitans and Macrocystis pyrifera with different molecular weights and mannuronic/guluronic ratios, M/G, were used as gel matrixes in order to obtain magnetic nanostructured composites. Magnetic nanocrystalline particles of iron oxides were formed inside the alginate matrix by in situ alkaline oxidation of ferrous ions. The magnetic materials obtained were subjected to several oxidative cycles and the increment in iron content was determined after each cycle. X-ray diffraction, magnetometry and Mössbauer spectroscopy were used to examine the materials. The high magnetic response, the absence of hysteresis, and the centered paramagnetic doublet in the Mössbauer spectra indicate the presence of nanocrystalline particles with a superparamagnetic behavior. X-ray diffractograms show peaks that correspond to maghemite. After the first cycle, Sargassum had four times the magnetic response of Macrocystis, which had more than twice the M/G ratio.

In situ synthesis of ferrites in ionic and neutral cellulose gels

Abstract The filaments of a tyrecord rayon were modified by two methods to enhance the degree of swelling of the material: (1) carboxyl and sulfonic acid substituents were introduced into the rayon and (2) the filaments were swollen in sodium hydroxide. The water-swollen filaments were rendered magnetic by in situ synthesis of ferrites and the resulting magnetic filaments were characterized by transmission electron microscopy (TEM) and vibrating sample magnetometry (VSM). Two other non-ionic, highly swollen cellulose gels were used as matrices for in situ synthesis of ferrites: a never-dried, wet-spun model cellulose filament and a never-dried bacterial cellulose membrane. TEM micrographs of thin cross-sections of the magnetic gels showed that the nanometre-sized ferrites were uniformly distributed whereas the treated rayon filaments had ferrites predominantly at the filament surface. All the materials were superparamagnetic as determined by VSM. However, a ferrimagnetic component was detected after several reaction cycles for the bacterial cellulose membrane by Mossbauer spectroscopy.

Formation and characterization of superparamagnetic cross-linked high amylose starch

A gelatinized cross-linked high amylose starch matrix with magnetic properties was synthesized via in situ formation of iron oxides inside the polymer matrix. Precipitation and multiple oxidation of ferrous ions were performed. The samples were observed using transmission and scanning electron microscopy, showing morphological changes in the magnetic and polymer phases. The iron content analysis revealed a decay from one oxidation cycle to the next one if no fresh ferrous solutions are added before the multiple oxidation. X-ray diffractograms, magnetization curves and Mossbauer spectra were also recorded for the characterization of the magnetic phase. The products exhibit superparamagnetic properties due to the presence of ferrimagnetic nanoparticles, although some other iron compounds are also present.

Structural and magnetic properties of RFe/sub 6/Ge/sub 6/ (R=Y, Gd, Tb, Er)

A series of RFe/sub 6/Ge/sub 6/ (B=Y, Gd, Tb, Er) compounds have been prepared by arc-melting. Structural studies using X-ray diffraction (XBD) show that all four as-cast alloys belong to the hexagonal P6/mmm space group with similar lattice parameters (for YFe/sub 6/Ge/sub 6/, a=5.12 /spl Aring/ and c=4.07 /spl Aring/). After annealing at 900/spl deg/C for two weeks, both YFe/sub 6/Ge/sub 6/ and TbFe/sub 6/Ge/sub 6/ transform into the orthorhombic Cmcm structure. Mossbauer spectroscopy and magnetisation measurements show that all of the alloys are antiferromagnetically ordered at room temperature. We find a single Fe crystallographic site, consistent with the RFe/sub 6/Ge/sub 6/ structures. Both the room temperature /sup 57/Fe hyperfine field and the Neel temperature (T/sub N/) are largely independent of the rare-earth, being 14.9(1) T and 487 K respectively for YFe/sub 6/Ge/sub 6/. >

Magnetic structure of GdBiPt: A candidate antiferromagnetic topological insulator

A topological insulator is a state of matter which does not break any symmetry and is characterized by topological invariants, the integer expectation values of nonlocal operators. Antiferromagnetism, on the other hand, is a broken symmetry state in which the translation symmetry is reduced and time reversal symmetry is broken. Can these two phenomena coexist in the same material? A proposal by Mong et al. [Phys. Rev. B 81, 245209 (2010)] asserts that the answer is yes. Moreover, it is theoretically possible that the onset of antiferromagnetism enables the nontrivial topology since it may create spin-orbit coupling effects which are absent in the nonmagnetic phase. The current work examines a real system, half-Heusler GdBiPt, as a candidate for topological antiferromagnetism. We find that the magnetic moments of the gadolinium atoms form ferromagnetic sheets which are stacked antiferromagnetically along the body diagonal. This magnetic structure may induce spin-orbit coupling on band electrons as they hop perpendicular to the ferromagnetic sheets.

Flat-plate single-crystal silicon sample holders for neutron powder diffraction studies of highly absorbing gadolinium compounds

The extreme absorption cross section of natural gadolinium has so far precluded routine neutron diffraction work on its alloys and compounds. However, it is shown here that an easily constructed flat-plate sample holder with silicon single-crystal windows can be used to place a thin layer of material in a neutron beam and obtain Rietveld refinement quality diffraction data in a modest time. The flat-plate geometry uses a large area to compensate for the necessarily thin sample. Demonstration data are presented on two intermetallic compounds, Sm3Ag4Sn4 and Gd3Ag4Sn4, and it is shown that both structural and magnetic information can be derived from the diffraction patterns. By working at a wavelength of 2.37 A, it is possible to observe the low-Q diffraction peaks associated with magnetic ordering. This simple methodology should now enable routine measurements on even the most highly absorbing materials.

The magnetic structure of

We have determined the magnetic structure of the Fe sublattice in by high-resolution neutron powder diffraction. The crystal space group is Cmcm and the magnetic space group is . The Fe modes are and at the 8d, 8e and 8g sites, respectively. The easy direction of magnetization is [100] and the propagation vector is [010].

Formation of high pressure phases in rapidly quenched Fe‐Nd alloys

Fe100−xNdx amorphous ribbons were obtained for compositions with 25<x<50, and partially amorphous ribbons for all other compositions.The amorphous phases were magnetically ordered with Curie temperatures ranging from 421 to 493 K. During crystallization, three metastable phases (M1, M2, and M3) were formed. X‐ray structural studies together with Mossbauer and thermomagnetic measurements suggest that the M1 phase is Fe23Nd6 (Mn23Th6 structure) with lattice parameter 1.152 nm and a Curie temperature of 515 K. The M2 phase is identified as Fe2Nd(Cu2Mg structure) with a lattice parameter of 0.745 nm and a Curie temperature of 567 K. The M1 and M2 phases transform to α‐Fe and Nd2Fe17 at high temperatures (≥1000 K). The M3 phase is present in the as‐quenched ribbons with x≥60 as well as in all crystallized ribbons. Structural data show that it is γ‐Nd, an fcc form of Nd. All three nonequilibrium structures are high pressure phases which are often formed during rapid solidification and/or crystallization of amor...

Low-background single-crystal silicon sample holders for neutron powder diffraction

Neutron diffraction measurements on weakly scattering or highly absorbing samples may demand custom mounting solutions. Two low-background sample holders based on inexpensive single-crystal silicon are described. One uses a conventional cylindrical geometry and is optimized for weakly scattering materials, while the other has a large-area flat-plate geometry and is designed for use with highly absorbing samples. Both holders yield much lower backgrounds than more conventional null-matrix or null-scattering materials and are essentially free from interfering Bragg peaks.