The use of chemical solution routes to form inorganic thin films is a relatively new method which represents an alternative to vapor phase routes. The present study involves the use of a chemical solution route, the decomposition of metal carboxylates, to prepare bismuth thin films of controlled porosity. Such morphologies offer the opportunity to disrupt phonon transport without greatly affecting electrical conductivity and bismuth represents a well known system in which to investigate these effects. Porous bismuth thin films have been prepared using bismuth 2-ethylhexanoate (Bi[OOCCH(C2 H5 )C4 H9 ]3 ) as the precursor in a solvent of 2-methyl- 1-propanol. The solution is deposited on glass, Kapton, silicon, alumina or magnesia substrates by spin coating and heated to between 250–300°C in hydrogen. Heat treatment temperature and time are important for controlling film microstructure as both pore volume (25 to 50%) and preferred orientation depend upon heat treatment conditions. Bismuth films (62 nm thick) with 32% porosity exhibit conductivities in the range of 150 S/cm with Seebeck coefficients comparable to that of bulk materials.

We investigate the effect of Fe substitution for Co on the lattice thermal conductivity of CoSb3 skutterudites. The polycrystalline materials are formed from uniaxially hot-pressed powders. Three alloys were prepared with 0, 3% and 10% Fe, respectively. Thermal conductivity measurements were made between 80 K to 450 K. The lattice thermal conductivity of 10%Fe:CoSb3 is approximately two times smaller than the lattice thermal conductivity of CoSb3 over the entire temperature range. This effect cannot be accounted for by the phenomenlogical theory considering only the mass difference and strain field due to the alloying (Fe) atom. Other phonon scattering mechanisms are discussed. Comparison is made with the partially-filled skutterudite alloy, La0.65 Fe2.8 Co1.1 Sb12.

We have developed a variety of templating processes for the fabrication of three-dimensionally periodic, nanostructured thermoelectrics from opal and inverse opal matrices. These opals and inverse opals are periodic at optical wavelengths and have extremely high interfacial area. It was hoped that scattering processes at the interface between opal and infiltrated thermoelectric material would increase the thermoelectric figure of merit (ZT) by having a greater effect on phonon-mediated (lattice) thermal conductivity than on electronic conductivity. We provide the first demonstration that this approach can increase ZT by evaluating a simple prototype system: bismuth infiltrated into porous SiO/sub 2/ opal. We find a larger fractional decrease in thermal conductivity than for electrical conductivity (relative to bulk polycrystalline Bi). Since the thermopower is little changed, the overall effect we observe is as much as a two-fold increase of ZT compared with that for polycrystalline bulk bismuth. However, the observed ZT is much smaller than for single crystal bismuth.

Crystallographic changes in a palladium wire cathode were monitored in situ, as deuterium was electrochemically deposited on the surface and diffused radially into the wire. Initially, the win was pure Pd. A constant electrolysis current density of 1 mA/cm(2) was maintained and D slowly diffused into the wire. As the D concentration increased, the wire transformed from pure Pd, to the alpha phase, and finally into the beta phase. This reversible phase transformation begins on the surface and progresses radially inward. During the experiment, x-ray-diffraction data were collected from a volume element of about 180 pi. This volume element was systematically moved in 50-mu m steps from the edge to the center of a 1.0 mm diameter Pd wire. Throughout the course of the experiment, the bulk value of x in PdDx, as determined from simultaneous measurements of the electrical resistivity, increased from 0 to similar to 0.72. For each setting of the volume element, a monotonic increase in the volume of the a phase was observed, until the material entered the two-phase region. Once the beta phase appeared, the volumes of both phases decreased slightly with continued loading. The integrated intensities of diffraction peaks from each phase were used in conjunction with the known phase diagram to estimate the rate of compositional change within the volume element. The diffusion rate for the solute atoms was estimated to be 57 +/- 8 nm/s, based on the temporal and spatial dependence of the integrated intensities of the diffraction peaks from each phase. These data also were used to evaluate the time dependence of the concentration of the solute atoms partial derivative c/partial derivative t and their diffusivity D. The value of partial derivative c/partial derivative t increased linearly from 6.2x 10(-5) s(-1) at the surface, to 7.6X10(-5) s(-1) midway into the wire. D was computed to be (3.1+/-1.0) x10(-11) m(2)/s when the transition began at r=250 mu m; 2 ks later it had decreased to (2.1+/-0.3) x 10(-11) m(2)/s. This may be due to the fact that the volume of the beta lattice also decreased during this period. [S0163-1829(98)01846-3].

When a new and promising thermoelectric material is discovered, an effort is undertaken to improve its ’figure of merit’. If the effort is to be more efficient than one of trial and error with perhaps some "rule of thumb guidance" then it is important to be able to make the connection between experimental data and the underlying material characteristics, electronic and phononic, that influence the figure of merit. Transport and fermiology experimental data can be used to evaluate these material characteristics and thus establish trends as a function of some controllable parameter, such as composition. In this paper some of the generic-materials characteristics, generally believed to be required for a high figure of merit, will be discussed in terms of the experimental approach to their evaluation and optimization. Transport and fermiology experiments will be emphasized and both will be outlined in what they can reveal and what can be obscured by the simplifying assumptions generally used in their interpretation.

The PrBa2Cu3O7-delta (Pr 123) system is known to exhibit antiferromagnetic ordering of the Cu spins in the Cu-O planes and chains, in addition to the observed antiferromagnetism due to the ordering of the Pr ions. The antiferromagnetism of the Cu-O planes is generally associated with the insulating nature of transport in Pr 123 and electrical conduction is believed to result through ''hopping'' transport between disordered Cu-O chain segments. However, the possible effects of magnetic ordering in the Cu-O chains on transport properties have not been discussed. If the Cu-O chains play a dominant role in the electrical transport (as suggested by many of the popular theories), then the magnetic ordering of the chains should also be seen in the transport properties. We present magnetization and magnetotransport measurements on single crystal Pr 123 samples which suggest that the magnetism associated with the Cu-O chains does indeed mediate transport in this system. These results confirm that the chains play a significant role in the transport properties and underscore the need for further study of the interplay between magnetism and transport in Pr 123.

We have measured the electrical resistivity and thermoelectric power of commercial vapor-grown carbon fibers and fiber composites from 4 K to 300 K. Post-growth heat treatment decreased the resistivity and moderately increased the magnitude of the thermopower. Thermoelectric measurements on fiber composites showed that the host material altered the resistivity but had little effect on the thermopower. Thermopower data suggest that the fibers in the composite material have a smaller degree of graphitization than isolated fibers.

We have measured the electrical resistivity, rho, thermoelectric power, alpha, and thermal conductivity, kappa, of the skutterudite material IrSb3 in a temperature range from 300 down to 4 K. It is found that the electrical resistivity and thermopower decrease monotonically as the temperature is reduced to 50-60 K. Below approximately 60 K the resistivity rises in a semiconducting manner. It appears the thermopower exhibits a large phonon drag peak at around 20 K and then falls towards zero. The thermal conductivity increases rapidly as the temperature is decreased with a maximum at around 20 K, corresponding to the peak in the thermopower. We will discuss these results and compare them to higher temperature data from G. A. Slack and V. G. Tsoukala [(lrSb3) J. Appl. Phys. 76, 1635 (1994)]. We have also measured some of the so-called ''filled skutterudites,'' Ir4LaGe3Sb9, Ir4NdGe3Sb9 and Ir(4)SaGe(3)Sb(9). The thermoelectric properties of these materials are considerably different than those of the unfilled sample. The thermopower is considerably lower and the resistivity is a factor of 2-4 times higher than the unfilled sample at room temperature. The thermal conductivity is markedly reduced by the filling, as much as a factor of 20 reduction for some of the systems. (C) 1996 American Institute of Physics.

For at least thirty years most of the generic features desirable in a material to have potential for a high thermoelectric figure of merit have been known and generally not disputed. Most of these involve some aspect of the electronic structure. The latter can be measured or at least qualitatively evaluated using traditional transport and fermiology experiments. These include temperature dependent electrical resistivity, low and high field Hall effect and magnetoresistance, and the Shubnikov-de Haas (or equivalently de Haas-van Alphen) effect. In this paper the theoretical basis for this kind of an evaluation will be given, with emphasis on the semimetallic class of materials where most thermoelectrics are found but which present their own problems. The four experimental techniques mentioned above will be described and some of the subtleties of the data reduction and analysis illustrated.

Polycrystalline samples of Ir4LaGe3Sb9, Ir4NdGe3Sb9, and Ir4SmGe3Sb9 have been made by hot isostatic pressing of powders. The lattice thermal conductivity of these filled skutterudites is markedly smaller than that of IrSb3; thus, void filling shows promise as a method for improving the thermoelectric properties of these materials. We present the lattice thermal conductivity of these filled skutterudites in an effort to quantify the impact of void filling in this structure. It is believed that the atoms ‘‘rattle’’ in the voids of the structure and therefore interact with a broad spectrum of lattice phonons, reducing their mean free paths substantially below that in the ‘‘unfilled’’ skutterudites. An additional phonon scattering mechanism is caused by phonon-stimulated transitions between the low-lying energy levels of the 4f electron configurations in the case of Nd3+ and Sm3+. Magnetic susceptibility and Hall-effect measurements are also presented.

The ${\mathrm{PrBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (Pr 123) system is known to exhibit antiferromagnetic ordering of the Cu spins in the Cu-O planes and chains, in addition to the observed antiferromagnetism due to the ordering of the Pr ions. The antiferromagnetism of the Cu-O planes is generally associated with the insulating nature of transport in Pr 123 and electrical conduction is believed to result through ``hopping'' transport between disordered Cu-O chain segments. However, the possible effects of magnetic ordering in the Cu-O chains on transport properties have not been discussed. If the Cu-O chains play a dominant role in the electrical transport (as suggested by many of the popular theories), then the magnetic ordering of the chains should also be seen in the transport properties. We present magnetization and magnetotransport measurements on single crystal Pr 123 samples which suggest that the magnetism associated with the Cu-O chains does indeed mediate transport in this system. These results confirm that the chains play a significant role in the transport properties and underscore the need for further study of the interplay between magnetism and transport in Pr 123.

We have measured the electrical resistivity, /spl rho/, and the thermopower, S, of PrBa/sub 2/Cu/sub 3-x/Ga/sub x/O/sub 7-/spl delta// in a temperature range from 300 K down to approximately 100 K. Samples with varying amounts of Ga substitution (x=0.0, 0.2, 0.4, 0.6, and 0.8) were studied, some of which were annealed in flowing O/sub 2/ for one week at 700/spl deg/C. At room temperature the resistivity of the material increases by several orders of magnitude as Ga is substituted for the Cu. The thermopower increases from 100 /spl mu/V/K for x=0.0 to approximately 300 /spl mu/V/K when x=0.2. The value does not continue to increase as more Ga is substituted. The resistivity increases as the temperature is lowered, while the thermopower exhibits a complicated temperature dependence. Annealing the material in O/sub 2/ decreases the resistivity, but does not change the room temperature value of the thermopower. Although the resistivity may follow a 3D variable range hopping expression as a function of temperature, the thermopower does not. The thermopower may show indications of magnetic transitions. The power factor (=S/sup 2///spl rho/) at room temperature is greatest for x=0.0 with a value of 10/sup -1/ /spl mu/W/(K/sup 2/-cm), it decreases over several orders of magnitude as Ga is substituted. It also decreases as the temperature is lowered. We conclude that the system PrBa/sub 2/Cu/sub 3-x/Ca/sub x/O/sub 7-/spl delta// is probably not going to be useful as a thermoelectric material, even with other substitutions such as Y for Pr.

The elastic properties of individual vapor-grown carbon fibers have been studied via the low-strain, vibrating-reed technique. The average Young's modulus is found to be 680 GPa, with some indication that the intrinsic modulus of CVD carbon may be much higher. This value exceeds those found by earlier high-strain, static-pulling methods. Although the amplitude of vibrational response for these fibers is linear in the driving force, the shape of the frequency response deviates from Lorentzian as the amplitude increases. This behavior has not been previously reported in other materials. Possible explanations based on the fibers' microstructure are discussed

The results of magnetoresistance and magnetization measurements taken on a single crystal of PrBa2Cu3O7-delta are reported. Among the family of materials known as the 1-2-3 compounds, only Pr 1-2-3 exhibits non-metallic transport and is not superconducting. Our results suggest that hybridization of Pr orbitals with Cu-O valence states may be responsible for the nonmetallic transport properties of Pr 1-2-3.

The results of magnetoresistance and magnetization measurements taken on a single crystal of PrBa2Cu3O7−δ are reported. Among the family of materials known as the 1-2-3 compounds, only Pr 1-2-3 exhibits non-metallic transport and is not superconducting. Our results suggest that hybridization of Pr orbitals with Cu-O valence states may be responsible for the nonmetallic transport properties of Pr 1-2-3.

We have used the vibrating-reed technique to investigate the magnetoelastic properties of NbSe 3 in an attempt to detect any contribution to stiffness from magnetic-field-assisted normal-to-charge-density-wave (CDW) carrier conversion below the material's lower CDW transition. For a magnetic-field transverse to the reed, we find the resonant frequency to decrease quadratically with field strength. However, the magnitude of this change is too large to be associated with carrier conversion. We can attribute the effect to the interaction between the field and the material's anisotropic magnetic susceptibility. This phenomenon is large enough to obscure attempts to detect stiffness contribution from carrier conversion. However, there exists the possibility of exploiting this effect to study the CDW through its influence on the magnetic susceptibility.

Magnetic fields may alter the flexural resonant frequencies of vibrating reeds that possess anisotropic magnetic susceptibility. The magnitude of the effect is a quadratic in the field strength. It is also a function of field orientation, the reed's aspect ratio and Young's modulus, the vibrational mode, and the magnetic susceptibility tensor. This phenomenon can provide a direct and extremely sensitive measure of magnetic anisotropy.

It is known that a stress-induced phase transition exists in TaSe3.[1, 2] We have simultaneously measured the stress, strain and resistance in TaSe3 samples through this phase transition. We have performed these measurements over a range of temperature from 25K to 300K We have also evaluated Young's modulus, Y, over this same range and find values which are comparable with those obtained for TaS3 and NbSe3. The modulus appears to have two levels; a high stress and low stress value which is consistent with the origin of the transition being structural in nature. The dramatic electronic properties that this material exhibits under stress are discussed in terms of this structural transition.