Ceramic-metal composites for heat exchangers in concentrated solar power plants.

The efficiency of generating electricity from heat using concentrated solar power plants (which use mirrors or lenses to concentrate sunlight in order to drive heat engines, usually involving turbines) may be appreciably increased by operating with higher turbine inlet temperatures, but this would require improved heat exchanger materials. By operating turbines with inlet temperatures above 1,023 kelvin using closed-cycle high-pressure supercritical carbon dioxide (sCO2) recompression cycles, instead of using conventional (such as subcritical steam Rankine) cycles with inlet temperatures below 823 kelvin1-3, the relative heat-to-electricity conversion efficiency may be increased by more than 20 per cent. The resulting reduction in the cost of dispatchable electricity from concentrated solar power plants (coupled with thermal energy storage4-6) would be an important step towards direct competition with fossil-fuel-based plants and a large reduction in greenhouse gas emissions7. However, the inlet temperatures of closed-cycle high-pressure sCO2 turbine systems are limited8 by the thermomechanical performance of the compact, metal-alloy-based, printed-circuit-type heat exchangers used to transfer heat to sCO2. Here we present a robust composite of ceramic (zirconium carbide, ZrC) and the refractory metal tungsten (W) for use in printed-circuit-type heat exchangers at temperatures above 1,023 kelvin9. This composite has attractive high-temperature thermal, mechanical and chemical properties and can be processed in a cost-effective manner. We fabricated ZrC/W-based heat exchanger plates with tunable channel patterns by the shape-and-size-preserving chemical conversion of porous tungsten carbide plates. The dense ZrC/W-based composites exhibited failure strengths of over 350 megapascals at 1,073 kelvin, and thermal conductivity values two to three times greater than those of iron- or nickel-based alloys at this temperature. Corrosion resistance to sCO2 at 1,023 kelvin and 20 megapascals was achieved10 by bonding a copper layer to the composite surface and adding 50 parts per million carbon monoxide to sCO2. Techno-economic analyses indicate that ZrC/W-based heat exchangers can strongly outperform nickel-superalloy-based printed-circuit heat exchangers at lower cost.

The Influence of Doping on the Optoelectronic Properties of PbS Colloidal Quantum Dot Solids.

We report on an extensive spectroscopic investigation of the impact of substitutional doping on the optoelectronic properties of PbS colloidal quantum dot (CQD) solids. N-doping is provided by Bi incorporation during CQD synthesis as well as post-synthetically via cation exchange reactions. The spectroscopic data indicate a systematic quenching of the excitonic absorption and luminescence and the appearance of two dopant-induced contributions at lower energies to the CQD free exciton. Temperature-dependent photoluminescence indicates the presence of temperature-activated detrapping and trapping processes of photoexcitations for the films doped during and after synthesis, respectively. The data are consistent with a preferential incorporation of the dopants at the QDs surface in the case of the cation-exchange treated films versus a more uniform doping profile in the case of in-situ Bi incorporation during synthesis. Time-resolved experiments indicate the presence of fast dopant- and excitation-dependent recombination channels attributed to Auger recombination of negatively charged excitons, formed due to excess of dopant electrons. The data indicate that apart from dopant compensation and filling of dopant induced trap states, a fraction of the Bi ionized electrons feeds the QD core states resulting in n-doping of the semiconductor, confirming reported work on devices based on such doped CQD material.

Förster resonant energy transfer from an inorganic quantum well to a molecular material: Unexplored aspects, losses, and implications to applications.

A systematic investigation of Förster resonant energy transfer (FRET) is reported within a hybrid prototype structure based on nitride single quantum well (SQW) donors and light emitting polymer acceptors. Self-consistent Schrödinger-Poisson modeling and steady-state and time-resolved photoluminescence experiments were initially employed to investigate the influence of a wide structural parameter space on the emission quantum yield of the nitride component. The optimized SQW heterostructures were processed into hybrid structures with spin-casted overlayers of polyfluorenes. The influence of important unexplored aspects of the inorganic heterostructure such as SQW confinement, content, and doping on the dipole-dipole coupling was probed. Competing mechanisms to the FRET process associated with interfacial recombination and charge transfer have been studied and their implications to device applications exploiting FRET across heterointerfaces have been discussed.

Concentration and excitation effects on the exciton dynamics of poly(3-hexylthiophene)/PbS quantum dot blend films.

The dynamics of photoexcitations in hybrid blends of poly(3-hexylthiophene) (P3HT) conjugated polymer donor and oleic-acid capped lead sulfide (PbS) quantum dot (QD) acceptors of different concentrations-for light harvesting applications-were investigated using time-resolved transmission and photoluminescence spectroscopies. Following excitation at 400 nm and probing in the 500-1000 nm region, we find that geminate excitation recombination in the blend of P3HT/PbS QDs dominates the transient decays at sub-ns times while intermaterial interactions such as charge transfer processes appear at longer times in the 1-50 ns regime. For the hybrid blend films with lower QD concentrations (<67% wt), polymer exciton recombination dominates the overall transient absorption signal. For higher QD contents, QD state relaxation effects become visible. Excitation density studies reveal the presence of linear exciton relaxation effects in the P3HT region while carrier decay for films with high PbS QD concentration is influenced by QD Auger recombination. Time-resolved luminescence shows that electron transfer from the P3HT/PbS QDs appears relatively inefficient in comparison to the geminate recombination, while hole transfer competes favorably to intrinsic QD recombination.

White light emission via cascade Förster energy transfer in (Ga, In)N quantum well/polymer blend hybrid structures.

We have studied the room-temperature non-radiative energy transfer processes in hybrid structures composed of (Ga, In)N/GaN single quantum wells and semiconducting polymer blend films placed in nanometre-scale proximity. The blends consist of three polyfluorene materials with concentrations adjusted so that they emit white light. Power-dependent photoluminescence (PL) measurements are used to investigate the process of energy transfer from the quantum wells to the different components of the polymer blend. We show that energy distribution among the hybrid structures involves competition between nanoscale range non-radiative energy transfer processes from the inorganic well to the polymer components and within the blend itself.

Reduction of spin injection efficiency by interface defect spin scattering in ZnMnSe/AlGaAs-GaAs spin-polarized light-emitting diodes.

We report the first experimental demonstration that interface microstructure limits diffusive electrical spin-injection efficiency across heteroepitaxial interfaces. An inverse correlation be-tween spin-polarized electron injection efficiency and interface defect density is demonstrated for ZnMnSe/AlGaAs-GaAs spin-polarized light-emitting diodes that exhibit quantum well spin polarizations up to 85%. A theoretical treatment shows that the suppression of spin injection due to interface defects results from the contribution of the defect potential to the spin-orbit interaction, which increases the spin-flip scattering.