Anders Bentien

Colossal Seebeck coefficient in strongly correlated semiconductor FeSb2

For more than a decade strongly correlated semiconductors and Kondo insulators have been considered as potential thermoelectric materials. Such materials have large d- or f-character of the electronic band structure close to the Fermi level that theoretically leads to Seebeck coefficients (S) with large magnitudes. In this work it is shown for the first time that the strongly correlated semiconductor FeSb2 exhibits a colossal Seebeck coefficient of ~−45000 μVK−1 at 10 K. The thermoelectric power factor PF=S2·ρ−1, where ρ is the electrical resistivity, reaches a record high value of ~2300 μWK−2 cm−1 at 12 K and is 65 times larger than that of the state-of-the-art Bi2Te3-based thermoelectric materials. However, due to a large lattice thermal conductivity the dimensionless thermoelectric figure of merit is only 0.005 at 12 K. Nonetheless, the potential of FeSb2 as a future solid-state thermoelectric cooling device at cryogenic temperatures is underlined.

Nonstoichiometry and chemical purity effects in thermoelectric Ba8Ga16Ge30 clathrate

Zone melting purification experiments have been carried out on the clathrate, Ba8Ga16Ge30. The impurities present have been identified and their approximate concentrations measured. Trace impurities were determined to be approximately 240 parts per million (ppm) in the most impure sample to 17 ppm in the most pure sample. The temperature-dependent Seebeck coefficient, thermal conductivity, and electrical conductivity are reported as a function of sample purity as well as the room-temperature Hall coefficient. Microprobe analysis suggests that the samples are nonstoichiometric with excess Ge relative to Ga, and there are indications of the presence of defects. Single-crystal x-ray investigations as well as synchrotron powder diffraction measurements support the presence of defects, but the x-ray data cannot accurately determine the relative amounts of Ga and Ge. Band-structure calculations in the generalized gradient approximation show that the measured Hall and Seebeck coefficients are consistent with a d...

Are type-I clathrates Zintl phases and 'phonon glasses and electron single crystals'?

Abstract We discuss to which extent the concepts of Zintl phases and of ‘phonon glasses and electron single crystals’ apply to type-I clathrates. In (β-) Eu 8 Ga 16 Ge 30 the presence of residual charge carriers appears to be related to a slight off-stoichiometry of the samples pointing to the validity of the Zintl concept in stoichiometric samples. The low and almost stoichiometry independent mobilities of (β-) Eu 8 Ga 16 Ge 30 , Sr 8 Ga 16 Ge 30 , and Ba 8 Ga 16 Ge 30 seriously question the validity of the ‘electron single crystal’ concept for type-I clathrates. The temperature dependence of the thermal conductivity of a Ba 8 Ga 16 Ge 30 single crystal indicates that tunneling states play a central role in producing ‘phonon glass’-like thermal conductivities.

Organic Redox Species in Aqueous Flow Batteries: Redox Potentials, Chemical Stability and Solubility

Organic molecules are currently investigated as redox species for aqueous low-cost redox flow batteries (RFBs). The envisioned features of using organic redox species are low cost and increased flexibility with respect to tailoring redox potential and solubility from molecular engineering of side groups on the organic redox-active species. In this paper 33, mainly quinone-based, compounds are studied experimentially in terms of pH dependent redox potential, solubility and stability, combined with single cell battery RFB tests on selected redox pairs. Data shows that both the solubility and redox potential are determined by the position of the side groups and only to a small extent by the number of side groups. Additionally, the chemical stability and possible degradation mechanisms leading to capacity loss over time are discussed. The main challenge for the development of all-organic RFBs is to identify a redox pair for the positive side with sufficiently high stability and redox potential that enables battery cell potentials above 1 V.

Solvothermal synthesis, multi-temperature crystal structures and physical properties of isostructural coordination polymers, 2C4H12N+[M3(C8H4O4)4]2−·3C5H11NO, M = Co, Zn

Two isostructural metal organic framework (MOF) structures have been synthesized by solvothermal methods and examined by single-crystal X-ray diffraction. A microcrystal of 2C4H12N+[Co3(C8H4O4)4]2-.3C5H11NO (1) was investigated at T = 120 K using synchrotron radiation. 2C4H12N+[Zn3(C8H4O4)4]2-.3C5H11NO (2) was investigated at multiple temperatures (T = 30, 100, 200 and 300 K) on a conventional diffractometer. The thermal expansion of the structure of (2) is anisotropic and along the a axis, which corresponds to the metal chain direction. The structures contain anionic frameworks with cations and solvent molecules trapped in the voids. The magnetic susceptibility (chi) and heat capacity (C(p)) have been measured from 1.8 to 350 K. Compound (1) orders ferromagnetically with a broad phase transition observed in C(p) at approximately 6 K. The magnetic moment reaches a value of 3 micro(B) per Co at 2 K in a magnetic field of 9 T, and a Curie-Weiss fit to chi(T) gives an effective moment (mu(eff)) of 4.2 mu(B) and a Weiss temperature (theta) of 23 K. The exchange mechanism for the magnetic coupling is suggested to involve the Co-O-Co bridges in the individual three-metal-atom subchains. The three-dimensional magnetism presumably is due to super-exchange through two out of the three unique C8H4O4 linker molecules, which have the carboxylate and benzene pi systems well aligned.

Tailoring Membrane Nanostructure and Charge Density for High Electrokinetic Energy Conversion Efficiency

The electrokinetic energy conversion (EKEC) of hydraulic work directly into electrical energy has been investigated in charged polymeric membranes with different pore charge densities and characteristic diameters of the nanoporous network. The membranes were synthesized from blends of nitrocellulose and sulfonated polystyrene (SPS) and were comprehensively characterized with respect to structure, composition, and transport properties. It is shown that the SPS can be used as a sacrificial pore generation medium to tune the pore size and membrane porosity, which in turn highly affects the transport properties of the membranes. Furthermore, it is shown that very high EKEC efficiencies (>35%) are encountered in a rather narrow window of the properties of the nanoporous membrane network, that is, with pore diameters of ca. 10 nm and pore charge densities of 4.6 × 10(2) to 1.5 × 10(3) mol SO3(-) m(-3) for dilute solutions (0.03 M LiCl). The high absolute value of the efficiency combined with the determination of the optimal membrane morphology makes membrane-based EKEC devices a step closer to practical applications and high-performance membrane design less empirical.

High electrokinetic energy conversion efficiency in charged nanoporous nitrocellulose/sulfonated polystyrene membranes.

The synthesis, characterization, and electrokinetic energy conversion performance have been investigated experimentally in a charged polymeric membrane based on a blend of nitrocellulose and sulfonated polystyrene. The membrane is characterized by a moderate ion exchange capacity and a relatively porous structure with average pore diameter of 11 nm. With electrokinetic energy conversion, pressure can be converted directly into electric energy and vice versa. From the electrokinetic transport properties, a remarkably large intrinsic maximum efficiency of 46% is found. It is anticipated that the results are an experimental verification of theoretical models that predict high electrokinetic energy conversion efficiency in pores with high permselectivity and hydrodynamic slip flow. Furthermore, the result is a promising step for obtaining efficient low-cost electrokinetic generators and pumps for small or microscale applications.

Low-temperature properties of CeRu4Sn6 from NMR and specific heat measurements: Heavy fermions emerging from a Kondo-insulating state

The combination of low-temperature specific-heat and nuclear-magnetic-resonance (NMR) measurements reveals important information of the ground-state properties of CeRu4Sn6, which has been proposed as a rare example of a tetragonal Kondo-insulator (KI). The NMR spin-latticerelaxation rate 1/T1 deviates from the Korringa law below 100 K signaling the onset of an energy gap ∆Eg1/kB ≈ 30 K. This gap is stable against magnetic fields up to 10 T. Below 10 K, however, unusual low-energy excitations of in-gap states are observed, which depend strongly on the field H . The specific heat C detects these excitations in the form of an enhanced Sommerfeld coefficient γ = C(T )/T : In zero field, γ increases steeply below 5 K, reaching a maximum at 0.1 K, and then saturates at γ ≈ 0.6 J/molK. This maximum is shifted to higher temperatures with increasing field suggesting a residual density of states at the Fermi level developing a spin gap ∆Eg2. A simple model, based on two narrow quasiparticle bands located at the Fermi level which cross the Fermi level in zero field at 0.022 states/meV f.u. can account qualitatively as well as quantitatively for the measured observables. In particular, it is demonstrated that fitting our data of both specific heat and NMR to the model, incorporating a Ce magnetic moment of μ = ∆Eg1/μ0H ≈ 1 μB , leads to the prediction of the field dependence of the gap. Our measurements rule out the presence of a quantum critical point as the origin for the enhanced γ in CeRu4Sn6 and suggest that this arises rather from correlated, residual in-gap states at the Fermi level. This work provides a fundamental route for future investigations into the phenomenon of narrow-gap formation in the strongly correlated class of systems.

Counter-ion Transport Number and Membrane Potential in Working Membrane Systems

In this work we use the general space-charge (SC) theory for a combined transport model of fluid and ion through cylindrical nanopores to derive equations for the membrane potential and counter-ion transport numbers. We discuss this approach for ion exchange membranes assuming aqueous domains as interconnected network of cylindrical pores. The transport number calculations from the SC theory are compared with the corresponding ones from the uniform potential (UP) and Teorell-Meyer-Sievers (TMS) models in the case of both zero and non-zero concentration gradient across the membrane and with an applied current density. By using this approach we suggest the optimal conditions for performing membrane potential experiments (i.e. choice of electrolyte and concentration difference) depending on an easily accessible membrane property, namely the volumetric charge density. We also theoretically describe a novel dynamic method to determine in a single experiment the membrane potential and membrane conductivity. To exemplify the use of the dynamic method we report the calculations based on typical operating conditions of the reverse electrodialysis process. The numerical results are presented in terms of the electrical potential difference versus the average pore radius and charge density. The resulting map is a useful tool for a rational design of an effective membrane morphology for a specific electrochemical application.

Two new cobalt–zinc orthophosphate monohydrates: hydrothermal synthesis, crystal structures and thermal investigation

Two new cobalt zinc orthophosphate hydrates with similar chemical formula, (CoxZn(1-x))3(PO4)2.H2O, but different composition and structure, have been prepared by systematic hydrothermal synthesis from the system nCo(CH3COO)2 : (1 -n)Zn(CH(3)COO)2 : 3.5H3PO4 : 2.1(CH3)2NH(CH2)3NH2:144H2O (0 </= n </= 1). The material Co(2.59)Zn(0.41)(PO4)2.H2O 1 has a three-dimensional structure that can be considered to be built from layers of edge-sharing CoO(6) octahedra joined by edge-sharing (Co/Zn)O(5) trigonal bipyramids, which also share edges with PO(4) tetrahedra. Compound 2, Co(0.72)Zn(2.28)(PO(4))(2).H(2)O, is isostructural with a known phase of Zn(3)(PO(4))(2).H(2)O: its structure contains corner-sharing (Zn/Co)O6 octahedra, (Zn/Co)O4 tetrahedra and PO4 tetrahedra, forming channels into which the coordinated water molecules project. Magnetic susceptibility measurements for 1 and 2 are consistent with the chemical compositions determined by the single-crystal X-ray analyses and with the presence of Co2+. The range for possible Co/Zn substitution in 1 and 2(assessed by EDX analysis) is relatively small: x lies in the range 0.74-0.80 (+/- 0.05) for 1 and 0.23-0.28 (+/- 0.05) for 2. Thermal investigation of 1 and 2 by thermogravimetry (TG), differential thermal analysis (DTA) and differential scanning calorimetry (DSC) shows that both materials transform to gamma-(CoxZn(1-x))3(PO4)2 when heated to 518 and 435 degrees C, respectively, with enthalpy changes for complete dehydration of DeltaH= 41.9 and 53.5 kJ mol(-1), respectively. Dehydration of 1 occurs in a single irreversible step, while that of 2 occurs over a greater temperature range and proceeds via several steps. A new phase, (CoxZn(1-x))3(PO4)2.0.27H2O, is formed when 2 is heated to 357 degrees C.