V. M. Browning

Electronic connection to the interior of a mesoporous insulator with nanowires of crystalline RuO2

Highly porous materials such as mesoporous oxides are of technological interest for catalytic, sensing and remediation applications: the mesopores (of size 2–50 nm) permit ingress by molecules and guests that are physically excluded from microporous materials. Connecting the interior of porous materials with a nanoscale or ‘molecular’ wire would allow the direct electronic control (and monitoring) of chemical reactions and the creation of nanostructures for high-density electronic materials. The challenge is to create an electronic pathway (that is, a wire) within a mesoporous platform without greatly occluding its free volume and reactive surface area. Here we report the synthesis of an electronically conductive mesoporous composite—by the cryogenic decomposition of RuO 4—on the nanoscale network of a partially densified silica aerogel. The composite consists of a three-dimensional web of interconnected (∼4-nm in diameter) crystallites of RuO2, supported conformally on the nanoscopic silica network. The resulting monolithic (RuO2∥SiO 2) composite retains the free volume of the aerogel and exhibits pure electronic conductivity. In addition to acting as a wired mesoporous platform, the RuO2-wired silica aerogel behaves as a porous catalytic electrode for the oxidation of chloride to molecular chlorine.

Magnetic and transport properties of radiation damaged La0.7Ca0.3MnO3.0 thin films

Magnetization and resistivity measurements are reported for a series of radiation damaged La0.7Ca0.3MnO3 pulsed laser deposited thin films. When plotted as a function of activation energy, trends in the electrical transport properties are similar to those exhibited in the magnetic properties. A sharp drop in both Tc and Tp in samples with activation energies greater than ∼110 meV suggests a “decoupling” of the magnetic and transport properties. The results suggest the magnetic order is no longer sufficient to delocalize the system of the extra disorder induced by the radiation damage.

THERMOELECTRIC PROPERTIES OF THE HALF-HEUSLER COMPOUND (Zr,Hf)(Ni,Pd)Sn

Recent measurements of the thermoelectric transport properties of a series of the half- Heusler compound ZrNiSn are presented. These materials are known to be bandgap intermetallic compounds with relatively large Seebeck coefficients and semimetallic to semiconducting transport properties. This makes them attractive for study as potential candidates for thermoelectric applications. In this study, trends in the thermoelectric power, electrical conductivity and thermal conductivity are examined as a function of chemical substitution on the various fcc sub-lattices that comprise the half-Heusler crystal structure. These results suggest that the lattice contribution to the thermal conductivity may be reduced by increasing the phonon scattering via chemical substitution. The effects of these substitutions on the overall power factor and figure-of-merit will also be discussed.

Characterization of transport and magnetic properties in thin film La0.67(CaxSr1−x)0.33MnO3 mixtures

We have grown thin films of (100)-oriented La0.67(CaxSr1−x)0.33MnO3 on (100) NdGaO3 substrates by off-axis sputtering. We have looked at the changes in the resistivity and magnetoresistance of the samples as the Ca/Sr ratio was varied. We find that as the calcium fraction is decreased, the lattice match to the substrate decreases, and the films become more disordered, as observed in transport measurements and the variation in Curie and peak resistance temperatures. We find a correlation between the temperature independent and T2 terms to the low temperature resistivity. The room temperature magnetoresistance exhibits a maximum as the peak temperature is increased by the substitution of Sr for Ca, and a change in the field dependence to the resistivity at room temperature is observed.

Interlayer coupling and enhanced coercivity in ferromagnetic/antiferromagnetic structures

Magnetization and ferromagnetic resonance measurements provide evidence of ferromagnetic coupling between ferromagnetic (F) layers separated by an intervening layer of antiferromagnetic (AF) NiO of thickness, tAF, where 7 nm<tAF<13 nm. Near 300 K, due to this coupling, the coercivity Hc and M–H loop squareness in F/AF multilayers increases as one increases the number of layers. The increased Hc and squareness have potential application in thin film magnets.

Chapter 2 Overview of measurement and characterization techniques for thermoelectric materials

Summary It is certainly no easy task to come up with highly reliable and accurate measurements, given all the parameters that must be measured to effectively determine ZT of a thermoelectric material. Many special errors and corrections for these thermoelectric materials must be taken into consideration. Thus, reports of materials that are only a few percent different from previous materials are not so convincing and of course are really not that much of a technological advancement in material performance. Many new researchers are coming into this field, and we remind them that much information concerning thermoelectric materials and the many excellent techniques for measuring their properties was developed in the late 1950s and 1960s. Careful investigation of the literature can provide valuable information and insight into the measurement of thermoelectric materials. The errors that can be made are sometimes very subtle and if we are to achieve much higher ZT materials, ZT ≈2–3, then we will have to be able to validate these numbers. Measuring standards, careful and accurate calibrations, and potential sharing of samples through “round robin” measurements are very important to eliminate erroneous information from being reported. There are, of course, other measurement techniques and apparatus that were not discussed in this overview, such as the slow AC technique for thermopower measurements ( Chaikin and Kwak, 1975 ) and other techniques that use slowly varying temperature methods ( Maldonado, 1992 ) just to name a couple of the more common methods. Researchers will have to judge for themselves which of the various techniques is the most appropriate for a given set of materials and for the equipment available to perform these measurements. We hope that this paper is informative both in terms of the issues involved in measuring thermoelectric materials and in terms of an extensive (yet incomplete) reference set of literature that is available. We hope it is especially useful to people new to the field of thermoelectric materials research, while it may serve as an elementary but important reminder to the established experts in the field. The accurate measurement and characterization of any new material will remain a challenge, and only correct care and attention to these measurements will rapidly advance the field of thermoelectric materials research.

X-ray photoemission characterization of La0.67(CaxSr1−x)0.33MnO3 films

The Curie temperature and x-ray photoemission spectra of thin films of La0.67(CaxSr1−x)0.33MnO3 have been studied as a function of the Ca/Sr ratio. The films were grown by off-axis cosputtering from individual targets of La0.67Ca0.33MnO3 (LCMO) and La0.67Sr0.33MnO3 (LSMO) onto (100) oriented NdGaO3 substrates. The films grow with a (100) orientation, with no other orientations observed by x-ray diffraction. For the alloy mixtures, the Curie temperature, TC, varies slowly as the Ca/Sr is decreased, remaining ≈300 K, while for the LCMO and LSMO films TC is 260 and 330 K, respectively. The Mn-O valence structure is composed of two dominant peaks, whose positions undergo a change as the Ca fraction is decreased. The core lines behave as linear combinations of lines from pure LCMO and LSMO.

Atomic structure, magnetism, and transport properties of damaged La0.67Ca0.33MnO3−δ processed via high-energy ball milling

A La0.67Ca0.33MnO3−δ alloy was produced via a low-temperature solid state reaction in a high-energy ball mill. The milled alloy is found to exhibit the paramagnetic-to-ferromagnetic (P–F) transition at 150 K. However, the widely reported insulator-to-metal (I–M) transition that usually accompanies the P–F transition is totally suppressed. Anneals at 500–1000 °C in air (t=8 h) are found to lower the resistivity but not restore the I–M transition. The suppression of the I–M transition is attributed to anion defects in the structure that have not been annihilated during heat treatments. Extended x-ray absorption fine structure is employed to track the evolution of the atomic structure around the Mn cations at various stages of processing.

Enhanced Jahn-Teller response induced by low-dose 10 MeV I+ irradiation of La0.7Ca0.3MnO3−δ films

The structural response of pulsed laser deposited La0.7Ca0.3Mn03−δ films to low-level 10 MeV I+ irradiation is correlated with magnetic and electronic properties. A series of annealed pulsed laser deposited films were subjected to ion irradiation at doses of 0.5–2.0 × 1013 10 MeV iodine ions/cm2. X-ray diffraction measurements show no measurable increase in the full width at half maximum values of major reflections before and after irradiation. Extended x-ray absorption fine structure analyses (EXAFS), employing both a model independent fitting using expanded cumulants as well as a nonlinear least squares multiple scattering model based on FEFF generated theoretical EXAFS data, were employed to measure local properties relative to the Mn ions. Increasing irradiation doses lead to an unambiguous evolution of MnO6 octahedra from highly symmetric (i.e., undistorted) to a clear bimodal structure in which the MnO6 octahedra are measured to undergo a c-axis stretching bearing resemblance to a Jahn-Teller distortion. Concomitantly, the metal to insulator transition temperature decreases more than 270 K and the magnetoresistance increases from 102% to 106%.

Ferromagnetic/antiferromagnetic structures with ferromagnetic interlayer coupling

Multilayer samples and structures composed of NiO(t)/Co(2.5 nm)/NiO(t)/Py(2.5 nm)/NiO(t) with t=6.6 and 13.2 nm exhibit ferromagnetic interlayer coupling below 400 K. A crossover is observed between the behavior near 300 K and at 10 K in ferromagnetic(F)/antiferromagnetic(AF) structures. Effects observed near 300 K, but not at 10 K, are an enhancement of Hc and the M–H loop squareness in F/AF multilayers as one increases the number of layers. A possible explanation for the crossover is given based on the domain wall thickness becoming less than the AF film thickness.