Juan E. Iglesias

Scaling the h-index for different scientific ISI fields

We propose a simple way to put in a common scale the h values of researchers working in different scientific ISI fields, so that the foreseeable misuse of this index for inter-areas comparison might be prevented, or at least, alleviated.

Optical properties of α-Fe2O3 microcrystals in the infrared

Since haematite ( alpha -Fe2O3) microcrystals are available with well defined shape (sphere, spindle and disc) and size homogeneity, the authors have been able to check the validity of the classical theory of absorption of radiation by small particles in the infrared. The agreement between observed and calculated infrared spectra is very good once particle aggregation is taken into consideration. Comparison between calculated and experimental IR spectra also affords a useful method for determination of the relative orientation of the crystallographic axes in the morphological coordinate frame.

The infrared dielectric properties of maghemite, γ-Fe2O3, from reflectance measurement on pressed powders

The infrared complex permittivity functions of three varieties of maghemite, γ-Fe2O3, having different degrees of vacancy ordering, have been determined from their IR reflectance spectra, measured at near to normal incidence on pressed powder pellets. The optical constants therefrom obtained have been verified by using them in the simulation of the corresponding absorption spectra for KBr-diluted pellets, and these are in excellent agreement with the experimental spectra. All calculations are based on a procedure for the estimation of the effective dielectric function of a mixture, which incorporates percolation features, recently developed by the authors.

Aggregation and Matrix Effects on the Infrared Spectrum of Microcrystalline Powders

Two pure phonon theories have been used for the interpretation of the infrared absorption spectra of powdered solids: the theory of absorption and scattering of light by small particles (TAS) and the theory of the average dielectric constant (TADC). The comparison between both theories shows that they are equivalent for particle sizes smaller than 2 μm and low particle concentration. However, when the particle concentration is significant, only TADC can be used to analyze the experimental spectra, thus obtaining a qualitative estimation of particle aggregation in a powder. Matrix effects were also observed in the experimental powder spectra, which can be accounted for by TADC.

Ionic conductivity and structural characterization of Na1.5Nb0.3Zr1.5(PO4)3 with NASICON-type structure

Abstract The NASICON-type Na1.5Nb0.3Zr1.5(PO4)3 was prepared by solid state reaction of Nb2O5 and the precursor γ-NaHZr(PO4)2 at 700 °C. The EPR spectra showed a signal with a g factor of 1.984 assigned to Nb (IV) species in octahedral oxygen environments. The X-ray powder diffraction pattern obtained with monochromatic radiation was indexed on the basis of a rhombohedral cell, the hexagonal parameters being aH = 8.8061(2) and c H = 22.7638(7) A . The Na+ ion conduction was measured by the complex impedance method (frequency range: 0.1–105 Hz; temperature range: 20–500 °C) on four pellets previously sintered at 450, 750, 900 and 1000 °C. The conductivity data are discussed in relation to the sintering temperature. An activation energy of 0.60 eV for the movement of Na+ ions in the NASICON framework has been found.

Single crystal aln fibers obtained by self-propagating high-temperature synthesis (SHS)

Single crystal AlN fibers [110] elongated up to 20 mm length and up to 40 μm diameter were obtained by self-propagating high-temperature synthesis (SHS) starting from a mixture of Al, AlN and NH4Cl under 100 atm. of N2. Two types of fibers were obtained: (i) long, thin fibers with smooth surfaces and diameter 20 μm) of “banderilla-type” having scores of crystalline ear-like projections aligned along generator lines on the fiber surfaces. The fibers grew by a vapor-solid mechanism in the axial geode-like region of the cylindrical AlN cake obtained after the SHS.

Effect of Particle Shape on the IR Reflectance Spectra of Pressed Powders of Anisotropic Materials

It is well known that the infrared (IR) properties of small particles diluted in a transparent matrix depend on particle shape,1±3 a dependence which is more notable for strongly anisotropic substances. However, to our knowledge, there is no extant report on the way particle shape could modify the IR spectra of dense aggregates. It is common to read about the use of empirical approximations to characterize re ̄ ectance spectra of powdered anisotropic ceramic materials (see, as a sample, Refs. 4 to 7, which extend over a period of 14 years), but none of them takes into account any information about particle shape. The success of such approximations is relative, and they are not applicable to many substances because the precise calculation of the average properties of heterogeneous systems is still an open problem, although many theoretical approximations have appeared in the literature over this century. In particular, different theories have been postulated in order to determine the electrical conductivity or the dielectric constant of aggregates of particles. The solution to the problem depends on adequately modeling the ``aggregate’ ’ , which can have different topological properties in different cases. In this work we show that particle shape is important in the computation of the average dielectric constant, even in systems consisting of high-density, pressure-compacted particles with air as a residual second component, a case where one could expect the properties of the ag-

Room temperature triclinic modification of NASICON-type LiZr2(PO4)3

Abstract The room temperature distortion of LiZr2(PO4)3 reacted at 1100°C and above, widely considered to be monoclinic, is proved here to be triclinic by detailed indexing of its powder X-ray diffraction pattern taken under good resolution conditions. This and other results seem to indicate that the low temperature distortion of most LiMe2IV(PO4)3 rhombohedral NASICON-type structures, is triclinic, while the homologous Na compounds are known to be monoclinic.

MODELING PARTICLE SIZE AND CLUMPING EFFECTS IN THE IR ABSORBANCE SPECTRA OF DILUTE POWDERS

The calculation of the IR absorbance of a powdered crystalline, ionic material, diluted in an appropriate IR-transparent matrix in a known proportion, is in principle possible, provided that the size and shape distributions of the particles, as well as the optical parameters of the bulk material, and those of the matrix, are known, and, in the case of the FT-IR technique, whether absolute calibration is possible, particularly at high absorbance values. In practice, a close match between observed and calculated absorbance is hindered by several factors intrinsic to the sample that are difficult to quantify, such as the degree of crystallinity of the particles and the very common clumping and aggregation of these particles, which produce a serious problem in the determination of the real filling factor of the composite sample. Some remedies to these problems are proposed in this paper and are applied to several well-known materials.

On the formation of mullite from kandites

Data are presented that support the explanation of the ∼980° C differential thermal analysis (DTA) exotherm of kandites in terms of transitional mullite (premullite) formation. The evolution of this material towards final 3∶2 mullite takes place with change in the tetrahedral aluminium/octahedral aluminium ratio, as evidenced by IR spectroscopy. The so-called mullite peak at 1250° C is shown to disappear when soaking experiments are carried out at lower temperatures, indicating that it is an effect of the dynamical character of DTA techniques.