Elizabeth Castillo-Martinez

Biscrolling nanotube sheets and functional guests into yarns.

Carbon nanotube sheets can support very large fractions of a second material, such as a superconductor or a catalyst. Multifunctional applications of textiles have been limited by the inability to spin important materials into yarns. Generically applicable methods are demonstrated for producing weavable yarns comprising up to 95 weight percent of otherwise unspinnable particulate or nanofiber powders that remain highly functional. Scrolled 50-nanometer-thick carbon nanotube sheets confine these powders in the galleries of irregular scroll sacks whose observed complex structures are related to twist-dependent extension of Archimedean spirals, Fermat spirals, or spiral pairs into scrolls. The strength and electronic connectivity of a small weight fraction of scrolled carbon nanotube sheet enables yarn weaving, sewing, knotting, braiding, and charge collection. This technology is used to make yarns of superconductors, lithium-ion battery materials, graphene ribbons, catalytic nanofibers for fuel cells, and titanium dioxide for photocatalysis.

Optical and morphological study of disorder in opals

An optical and morphological study has been carried out to understand the role of intrinsic defects in the optical properties of opal-based photonic crystals. By doping poly(methylmethacrylate) (PMMA) thin-film opals with larger polystyrene (PS) spheres, structural disorder has being generated perturbing the PMMA matrix periodicity. It is shown that this disorder dramatically affects the optical response of the system worsening its photonic properties. It has been found that the effect of doping is highly dependent not only on the concentration but also on the relative size of the dopant with reference to the matrix. Through a detailed scanning electron microscopy inspection, the sort of structural defects involved, derived from the different particle size used, has been characterized. A direct relationship between the observed optical response with the different perturbations generated in the lattice has been found. In addition, from this study it can be concluded that it is possible to grow high quality...

Highly Stable Cooperative Distortion in a Weak Jahn―Teller d2 Cation: Perovskite-Type ScVO3 Obtained by High-Pressure and High-Temperature Transformation from Bixbyite

A novel ScVO(3) perovskite phase has been synthesized at 8 GPa and 1073 K from the cation-disordered bixbyite-type ScVO(3). The new perovskite has orthorhombic symmetry at room temperature, space group Pnma, and lattice parameters a = 5.4006(2) Å, b = 7.5011(2) Å, and c = 5.0706(1) Å with Sc(3+) and V(3+) ions fully ordered on the A and B sites of the perovskite cell. The vanadium oxygen octahedra [V-O(6)] display cooperative Jahn-Teller (JT) type distortions, with predominance of the tetragonal Q(3) over the orthorhombic Q(2) JT modes. The orthorhombic perovskite shows Arrhenius-type electrical conductivity and undergoes a transition to triclinic symmetry space group P-1 close to 90 K. Below 60 K, the magnetic moments of the 4 nonequivalent vanadium ions undergo magnetic long-range ordering, resulting in a magnetic superstructure of the perovskite cell with propagation vector (0.5, 0, 0.5). The magnetic moments are confined to the xz plane and establish a close to zigzag antiferromagnetic mode.

Spinel to CaFe2O4 transformation: mechanism and properties of beta-CdCr2O4.

The CdCr(2)O(4) spinel transforms to a 10.6% denser new polymorph of the CaFe(2)O(4)-type structure at 10 GPa and 1100 degrees C. This new polymorph has a honeycomb-like structure because of double rutile-type chains formed by [Cr-O(6)] edge-shared octehedra. This crystal structure is prone to be magnetically frustrated and presents low-dimensional antiferromagnetism at 25 K < T < 150 K, accompanied by more complex interactions as the temperature decreases. These transitions are evidenced by magnetic susceptibility and heat capacity measurements. We also discuss a possible structural mechanism for the transformation.

Electron energy loss spectroscopy in ACrO3 (A = Ca, Sr and Pb) perovskites

We present the experimental ELNES spectra of the Cr-L2,3 and O-K edges in three high pressure synthesized perovskites, ACrO3 (A = Ca, Sr and Pb). A comparison of the experimental spectra against the theoretical calculations shows the influence of the A cation in the features of the spectrum. The Cr-L2,3 multiplet structure of these perovskites shows similar integrated intensity ratio () between them and with the CrO2, implying that all of them are indeed composed by Cr4+. But the O-K pre-edge observed in CrO2, typical of its d3L character (L-ligand hole), is not present in these perovskites. The effect of the A cation manifests itself in the O-K edge. The different features in the peak edges are caused by the change in the interaction between oxygen and the A cation from being via p orbitals (with Pb) to being via d orbitals (with Ca, Sr). The experimental spectra of the samples are reproduced well by the presented DFT calculations.

The A(II)Cr(IV)O3 (A=Sr, Ca, Pb) ‘simple’ perovskites. Structure and properties: magnetic structure of CaCrO3

The ‘simple’ perovskites ACr(IV)O3 (A=Sr, Ca, Pb) have been synthesized at high pressure and temperature. Their microstructure and properties are discussed. Specific heat and susceptibility measurements indicate an antiferromagnetic ordering at low temperature in CaCrO3, which is confirmed to be of the C-type by neutron diffraction. †This paper was presented at the XLVIth European High Pressure Research Group (EHPRG 46) Meeting, Valencia (Spain), 7–12 September, 2008.

Optical and morphological study of compound polymer opals

An optical and morphological study has been carried out to understand the role of intrinsic defects in opal-based photonic crystals. The inherent polydispersity in sphere size distribution yields imperfect crystallizations and worsens the photonic properties of these systems. By doping poly-(mehtymethacrylate) thin films opals with polystyrene spheres of larger size it is possible to study the disorder caused by the dopants and the negative influence in the optical response. In addition, it is feasible to grow mixed structures (alloys) with intermediate photonic properties by mixing spheres of different nature and the same size.