Thermal Transport and Thermoelectric Effect in Composites of Alumina and Graphene-Augmented Alumina Nanofibers.

The remarkable tunability of 2D carbon structures combined with their non-toxicity renders them interesting candidates for thermoelectric applications. Despite some limitations related to their high thermal conductivity and low Seebeck coefficients, several other unique properties of the graphene-like structures could out-weight these weaknesses in some applications. In this study, hybrid structures of alumina ceramics and graphene encapsulated alumina nanofibers are processed by spark plasma sintering to exploit advantages of thermoelectric properties of graphene and high stiffness of alumina. The paper focuses on thermal and electronic transport properties of the systems with varying content of nanofillers (1-25 wt.%) and demonstrates an increase of the Seebeck coefficient and a reduction of the thermal conductivity with an increase in filler content. As a result, the highest thermoelectric figure of merit is achieved in a sample with 25 wt.% of the fillers corresponding to ~3 wt.% of graphene content. The graphene encapsulated nanofibrous fillers, thus, show promising potential for thermoelectric material designs by tuning their properties via carrier density modification and Fermi engineering through doping.

Functionally Graded Tunable Microwave Absorber with Graphene-Augmented Alumina Nanofibers.

Graphene is currently attracting attention for radiation absorption particularly at gigahertz and terahertz frequencies. In this work, composites formed by graphene-augmented γ-Al2O3 nanofibers embedded into the α-Al2O3 matrix are tested for X-band absorption efficiency. Composites with 15 and 25 wt % of graphene fillers with shielding effectiveness (SE) of 38 and 45 dB, respectively, show a high reflection coefficient, while around the electrical percolation threshold (∼1 wt %), an SE of 10 dB was achieved. Furthermore, based on the dielectric data obtained for varying fractions of graphene-/γ-Al2O3-added fillers, a functionally graded multilayer is constructed to maximize the device efficiency. The fabricated multilayer offers the highest absorption efficiency of 99.99% at ∼9.6 GHz and a full X-band absorption of >90% employing five lossy layers of 1-3-5-15 and 25 wt % of graphene/γ-Al2O3 fillers. The results prove a remarkable potential of the fillers and various multilayer designs for broad-band and frequency-specific microwave absorbers.

Emergence of Metallic Conductivity in Ordered One-Dimensional Coordination Polymer Thin Films upon Reductive Doping.

The prospect of introducing tunable electric conductivity in metal-organic coordination polymers is of high interest for nanoelectronic applications. As the electronic properties of these materials are strongly dependent on their microstructure, the assembly of coordination polymers into thin films with well-controlled growth direction and thickness is crucial for practical devices. Here, we report the deposition of one-dimensional (1D) coordination polymer thin films of N,N'-dimethyl dithiooxamidato-copper by atomic/molecular layer deposition. High out-of-plane ordering is observed in the resulting thin films suggesting the formation of a well-ordered secondary structure by the parallel alignment of the 1D polymer chains. We show that the electrical conductivity of the thin films is highly dependent on their oxidation state. The as-deposited films are nearly insulating with an electrical conductivity below 10-10 S cm-1 with semiconductor-like temperature dependency. Partial reduction with H2 at elevated temperature leads to an increase in the electrical conductivity by 8 orders of magnitude. In the high-conductance state, metallic behavior is observed over the temperature range of 2-300 K. Density functional theory calculations indicate that the metallic behavior originates from the formation of a half-filled energy band intersecting the Fermi level with the conduction pathway formed by the Cu-S-Cu interaction between neighboring polymer chains.

Organic-Component Dependent Crystal Orientation and Electrical Transport Properties in ALD/MLD Grown ZnO-Organic Superlattices.

Two series of ZnO-organic superlattice thin films are fabricated with systematically controlled frequencies of monomolecular hydroquinone (HQ) or terephthalic acid (TPA) based organic layers within the ZnO matrix using the atomic/molecular layer deposition (ALD/MLD) technique. The two different organic components turn the film orientation to different directions and affect the electrical transport properties differently. While the TPA layers enhance the c-axis orientation of the ZnO layers and act as electrical barriers depressing the electrical conductivity even in low concentrations, adding the HQ layers enhances the a-axis orientation and initially increases the carrier concentration, effective mass, and electrical conductivity. The work thus demonstrates the intriguing but little exploited role of the organic component in controlling the properties of the inorganic matrix in advanced layer-engineered inorganic-organic superlattices.

Fe3Se4: a possible ferrimagnetic half-metal?

Half-metallic ferromagnets show 100% spin-polarization at the Fermi level and are ideal candidates for spintronic applications. Despite the extensive research in the field, very few materials have been discovered so far. Here, we present results of electronic band structure calculations based on density functional theory and extensive physical-property measurements for Fe3Se4 revealing signatures of half-metallicity. The spin-polarized electronic band structure calculations predict half-metallic ferrimagnetism for Fe3Se4. The electrical resistivity follows exponentially suppressed electron-magnon scattering mechanism in the low-temperature regime and show a magnetoresistance effect that changes the sign from negative to positive with decreasing temperature around 100 K. Other intriguing observations include the anomalous behavior of Hall resistance below 100 K and an anomalous Hall coefficient that roughly follows the ρ 2 behavior.

Flexible ε-Fe2O3-Terephthalate Thin-Film Magnets through ALD/MLD.

Pliable and lightweight thin-film magnets performing at room temperature are indispensable ingredients of the next-generation flexible electronics. However, conventional inorganic magnets based on f-block metals are rigid and heavy, whereas the emerging organic/molecular magnets are inferior regarding their magnetic characteristics. Here we fuse the best features of the two worlds, by tailoring ε-Fe2O3-terephthalate superlattice thin films with inbuilt flexibility due to the thin organic layers intimately embedded within the ferrimagnetic ε-Fe2O3 matrix; these films are also sustainable as they do not contain rare heavy metals. The films are grown with sub-nanometer-scale accuracy from gaseous precursors using the atomic/molecular layer deposition (ALD/MLD) technique. Tensile tests confirm the expected increased flexibility with increasing organic content reaching a 3-fold decrease in critical bending radius (2.4 ± 0.3 mm) as compared to ε-Fe2O3 thin film (7.7 ± 0.3 mm). Most remarkably, these hybrid ε-Fe2O3-terephthalate films do not compromise the exceptional intrinsic magnetic characteristics of the ε-Fe2O3 phase, in particular the ultrahigh coercive force (∼2 kOe) even at room temperature.

Isovalent substitution effects on thermoelectric transport properties of CoSbX (X = S, Se, Te) system.

We demonstrate a transition of the thermoelectric transport characteristics in the CoSbX (X = S, Se or Te) systems from a p -type semiconductor to metallic conductor with increasing size of the X constituent. From DFT calculations CoSbS is found as an indirect semiconductor with band-gap of 0.38 eV, while both CoSbSe and CoSbTe appear as metals. For the two metals, the calculations reveal two degenerate electron pockets (located near the U point for CoSbSe and near the T point for CoSbTe) and a hole pocket along the X-Γ-Y points. In line with the theoretical predictions, electrical transport measurements reveal semiconducting-type temperature dependence of resistivity and positive room-temperature Seebeck coefficient (+570 µV K-1) for CoSbS, and metallic-type temperature dependence for CoSbSe and CoSbTe with negative Seebeck coefficient (-14 and -7.5 µV K-1). The Hall coefficient is positive for CoSbS(Se) and negative for CoSbTe. Room-temperature charge carrier densities were estimated at 3 × 1018/~1021/~1022 cm-3 for CoSbS/CoSbSe/CoSbTe. Thermal conductivity is dominated by lattice rather than electronic contribution, the RT value being of the roughly same magnitude for all the three compounds. The temperature dependence of thermal conductivity bear resemblance to a typical semiconductor in the case of CoSbS and to a metallic alloy for CoSbSe and CoSbTe.

Negative Magnetoresistance in Amorphous Indium Oxide Wires.

We study magneto-transport properties of several amorphous Indium oxide nanowires of different widths. The wires show superconducting transition at zero magnetic field, but, there exist a finite resistance at the lowest temperature. The R(T) broadening was explained by available phase slip models. At low field, and far below the superconducting critical temperature, the wires with diameter equal to or less than 100 nm, show negative magnetoresistance (nMR). The magnitude of nMR and the crossover field are found to be dependent on both temperature and the cross-sectional area. We find that this intriguing behavior originates from the interplay between two field dependent contributions.

Thermoelectric properties of Cu and Cr disordered CuCrX2(X=S, Se): a first principles study.

The layered antiferromagnetic ACrX(2) -type compounds are currently highlighted as prominent material candidates for low- and intermediate-temperature thermoelectric (TE) applications. A key to attain the enhanced TE characteristics is to apply high-temperature sintering which presumably introduces some cation disorder. Here we present spin unrestricted density functional theory analysis of electronic band structures and TE properties of Cu and Cr disordered CuCrX(2)(X = S, Se) phases. A narrow band gap semiconductor to metal transition is observed on 8.3% Cr-site disorder for both the compounds, X = S and Se. The large p-type Seebeck coefficient realized in the metallic state for the Cr-disordered phases is the factor that makes these phases promising TE materials. These theoretical findings for the Cr-disordered phases are well in line with reported experimental data for electronic transport properties. Contrarily, the results revealed for the Cu-disordered phases do not agree with the experimental data. Hence the results of our theoretical analysis strongly point towards the Cr rather than the Cu disorder picture to explain the TE electronic transport characteristics of the high-temperature sintered phases of CuCrX(2)(X = S, Se).

First-principles study of layered antiferromagnetic CuCrX2 (X = S, Se and Te).

We present detailed electronic band-structure calculations for antiferromagnetic chromium compounds, CuCrX(2) (X = S, Se or Te), carried out using spin-polarized density functional theory within the generalized-gradient approximation (GGA). A narrow-band semiconductor-to-metal transition is observed upon replacement of S or Se by Te. The indirect bandgap is found at 0.58 eV and 0.157 eV for CuCrS(2) and CuCrSe(2), respectively. The results for our theoretical calculations are well in line with the electronic transport properties experimentally observed for CuCrS(2) and CuCrSe(2).