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The reflection of light from distributed microplatelets is an effective approach to creating color and controlling the optical properties in paints, security features, and optical filters. However, predictive tools for the design and manufacturing of such composite materials are limited due to the complex light-matter interactions that determine their optical response. Here, the optical reflectance of individual reflective microplatelets and of polymer-based composites containing these engineered platelets as an aligned, dispersed phase are experimentally studied and analytically calculated. Transfer-matrix calculations are used to interpret the effect of the platelet architecture, the number of platelets, and their size distribution on the experimentally measured reflectance of composites prepared using a previously established magnetic alignment technique. It is demonstrated that the reflectance of the composites can be understood as the averaged response of an array of Fabry-Perot resonators, in which the microplatelets act as semi-transparent flat reflectors and the polymer as cavity medium. By using an analytical model and computer simulations to describe the interaction of light with platelets embedded in a polymer matrix, this work provides useful tools for the design and fabrication of composites with tailored optical reflectance., Advanced Optical Materials, 11 (7), ISSN:2195-1071

The microstructural and optical reflectivity response of photonic SiO2/TiO2 nanomultilayers have been investigated as a function of temperature and up to the material system’s melting point. The nanomultilayers exhibit high, broadband reflectivities up to 1350 °C with values that exceed 75% for a 1 μm broad wavelength range (600–1600 nm). The optimized nanometer sized, dielectric multilayers undergo phase transformations from anatase TiO2 and amorphous SiO2 to the thermodynamically stable phases, rutile and cristobalite, respectively, that alter their structural morphology from the initial multilayers to that of a scatterer. Nonetheless, they retain their photonic characteristics, when characterized on top of selected substrate foils. The thermal behavior of the nanometer sized multilayers has been investigated by differential thermal analysis (DTA) and compared to that of commercially available, mm-sized, annealed powders. The same melting reactions were observed, but the temperatures were lower for the nm-sized samples. The samples were characterized using X-ray powder diffraction before DTA and after annealing at temperatures of 1350 and 1700 °C. The microstructural evolution and phase compositions were investigated by scanning electron microscopy and energy-dispersive X-ray spectroscopy measurements. The limited mutual solubility of one material to another, in combination with the preservation of their optical reflectivity response even after annealing, makes them an interesting material system for high-temperature, photonic coatings, such as photovoltaics, aerospace re-entry and gas turbines, where ultra-high temperatures and intense thermal radiation are present.

Porous carbon ablators offer cost-effective thermal protection for aerospace vehicles during re-entry into planetary atmospheres. However, the exploration of more distant planets requires the development of ablators that are able to withstand stronger thermal radiation conditions. Here, we report the development of bio-inspired porous carbon insulators with pore sizes that are deliberately tuned to enhance heat-shielding performance by increasing scattering of high-temperature thermal radiation. Pore size intervals that promote scattering are first estimated using an established model for the radiative contribution to the thermal conductivity of porous insulators. On the basis of this theoretical analysis, we identify a polymer additive that enables the formation of pores in the desired size range through the polymerization-induced phase separation of a mixture of phenolic resin and ethylene glycol. Optical and electron microscopy, porosimetry and mechanical tests are used to characterize the structure and properties of porous insulators prepared with different resin formulations. Insulators with pore sizes in the optimal scattering range reduce laser-induced damage of the porous structures by up to 42%, thus offering a promising and simple route for the fabrication of carbon ablators for enhanced thermal protection at high temperatures., Carbon, 195, ISSN:0008-6223

The microstructural and optical reflectivity response of photonic SiO2/TiO2 nanomultilayers have been investigated as a function of temperature and up to the material system’s melting point. The nanomultilayers exhibit high, broadband reflectivities up to 1350 °C with values that exceed 75% for a 1 μm broad wavelength range (600–1600 nm). The optimized nanometer sized, dielectric multilayers undergo phase transformations from anatase TiO2 and amorphous SiO2 to the thermodynamically stable phases, rutile and cristobalite, respectively, that alter their structural morphology from the initial multilayers to that of a scatterer. Nonetheless, they retain their photonic characteristics, when characterized on top of selected substrate foils. The thermal behavior of the nanometer sized multilayers has been investigated by differential thermal analysis (DTA) and compared to that of commercially available, mm-sized, annealed powders. The same melting reactions were observed, but the temperatures were lower for the nm-sized samples. The samples were characterized using X-ray powder diffraction before DTA and after annealing at temperatures of 1350 and 1700 °C. The microstructural evolution and phase compositions were investigated by scanning electron microscopy and energy-dispersive X-ray spectroscopy measurements. The limited mutual solubility of one material to another, in combination with the preservation of their optical reflectivity response even after annealing, makes them an interesting material system for high-temperature, photonic coatings, such as photovoltaics, aerospace re-entry and gas turbines, where ultra-high temperatures and intense thermal radiation are present. Supplementary Information The online version contains supplementary material available at 10.1007/s10853-021-06557-y.

A simple and thermally stable photonic heterostructure exhibiting high average reflectivity (⟨R⟩ ≈ 88.8%) across a broad wavelength range (920–1450 nm) is presented. The design combines a thin, hig...

Photonic additives have been investigated as a means to enhance the efficiency of thermal protection systems (TPS) against the adverse effects of thermal radiation. State-of-the-art TPS consist of carbon fibers embedded in a phenolic resin matrix. During operation, the TPS is consumed because it is exposed to an excess heat flux, a large fraction of which is due to thermal radiation. Here, we show that a properly modeled and designed additive-impregnated TPS can block a considerable part of this heat influx and quantify how different control parameters, in particular the additives’ amount, placement and alignment, influence the achieved photonic enhancement. More specifically, the intrinsic reflectivity of 8.5% of a conventional TPS can been improved to values exceeding 85% by controllably inserting additives, consisting of a Ta/[SiO2/TiO2]6 heterostructure, here referred to as Type 1, an ideal, optimized, high and broadband reflector. Nevertheless, even simple, commercially available additives composed of TiO2/Al2O3/TiO2, here referred to as Type 2, provide a high reflectivity enhancement with values of up to 76%, when used in larger quantities. The simulations of this work are based on the Monte Carlo Ray Tracing (MCRT) method. The MCRT simulation method has been validated against experiment, using the structure and experiments from a literature reference. Our analysis method allows one to design and model the performance of photonically enhanced TPS that operate in high-flux, radiative conditions, like those expected in future aerospace re-entry missions or next-generation, gas turbines and thermophotovoltaic plants and provides a viable option for efficiently enhancing a TPS. ISSN:1290-0729 ISSN:1778-4166

Metal-dielectric platelets are introduced as additives to heat protection systems. The platelets feature high reflectivity across 700 nm and thermal stability up to 1000°C. Impregnating aerospace heat shields improves thermal reflectivity by a factor 11., OSA Advanced Photonics Congress (AP) 2020 (IPR, NP, NOMA, Networks, PVLED, PSC, SPPCom, SOF), ISBN:978-1-943580-79-8

Nanowire-based solar cells opened a new avenue for increasing conversion efficiency and rationalizing material use by growing different III–V materials on silicon substrates. Here, we propose a multiterminal nanowire solar cell design with a theoretical conversion efficiency of 48.3% utilizing an efficient lateral spectrum splitting between three different III–V material nanowire arrays grown on a flat silicon substrate. This allows choosing an ideal material combination to achieve the proper spectrum splitting as well as fabrication feasibility. The high efficiency is possible due to an enhanced absorption cross-section of standing nanowires and optimization of the geometric parameters. Furthermore, we propose a multiterminal contacting scheme that can be fabricated with a technology close to standard CMOS. As an alternative we also consider a single power source with a module level voltage matching. These new concepts open avenues for next-generation solar cells for terrestrial and space applications., ACS Photonics, 2 (9), ISSN:2330-4022

A linear solar-tracking spectrum-splitting scheme for solar photovoltaics is investigated. The investigation is made by means of a numerical optimization and an analytical approach to predict conversion efficiency based on available experimental data. For a simple spectrum splitting scheme, a high conversion efficiency limit of 43% without and 48% with solar concentration is predicted for two single junction solar cells. It is also shown that efficiencies of at least 37% and 41% are to be expected if state of the art cells are used. The investigation was performed using a Fourier Modal Method electromagnetic solver based on the scattering matrix formalism combined with a numerical optimizer utilizing the Evolution Strategy.

Yttria stabilized zirconia (YSZ) thermal barrier coatings (TBC) can be destabilized by combustions products, formed due to either dirty fuel or dust ingested through the air intake. The origin of this destabilization is high-temperature interaction of YSZ with molten calcium-magnesium-alumino-silicate (CMAS) deposits. The destabilization of the YSZ changes the behavior of the TBC, finally leading to failure. As a part of this investigation, X-ray powder diffraction, scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX) studies of ex-service combustor tiles subjected to CMAS attack have been performed. Rietveld refinement of powder diffraction patterns is allowed for the quantitative analysis of phase transformations, chemical composition change of YSZ and reconstruction of the scheme of element migration during the interaction of YSZ with the CMAS deposit. The composition of the deposit that infiltrated the open pores of the YSZ was found to be different from the average deposit composition. Based on the results, some solutions to protect YSZ against CMAS attack are discussed.

Nickel-based IN738LC samples were built by selective laser melting (SLM). To evaluate the anisotropic mechanical behavior of IN738LC material due to layer-wise build up, specimens were built with their cylinder axis (loading direction) oriented either parallel to the building direction, or perpendicular to the building direction and at 45° to the laser scanning direction. After building up the specimens by SLM, they were investigated either under the ‘as-built’ condition or after heat treatment and compared to IN738LC cast material. The analysis of microstructural anisotropy in SLM made IN738LC specimens was done by using EBSD, EDX and X-ray texture analysis methods, and then correlated with anisotropic material behavior observed during tensile and creep testing at room temperature and 850 °C. All SLM samples possess the same general texture, with the majority of grains forming one single component of a cube texture with one of the cubic axes parallel to the building direction, and another cubic axis parallel to the laser scanning direction. The Young׳s modulus determined during tensile testing is significantly lower parallel to the building direction than perpendicular to the building direction, with the values for cast IN738LC material in between. Creep behavior of specimens with loading parallel to the building direction is superior compared to specimens with loading axis normal to the building direction. The anisotropy of Young׳s modulus was modeled based on the single crystal elastic tensor and the measured crystallographic preferred orientations, and compares well with the data from tensile tests.

This study aims at the theoretical examination of the anisotropy of thermal conductivity F = κ || / κ ⊥ that could be engineered in hard multilayer coatings, prepared using arc-evaporation. High values of F and thereby high lateral heat dissipation can reduce detrimental thermal gradients that emerge during cutting and friction processes in hard coatings. As coating deposition is widely done using arc-evaporation that leads to inclusion of droplets of different shapes and material in the coating, it is further evaluated to which extent F is be affected by these inclusions. The ideal continuous anisotropic medium with inclusions can be described using the effective medium Maxwell Garnett Approximation (MGA). A deposited multilayer structure presents a particular case with a limited number of layers inducing anisotropy and is simulated using the Finite Element Method (FEM) and compared to the idealized MGA predictions. The results show that the effect of droplets on the anisotropy F is strongly dependent on droplet shape, material and orientation. For spherical droplets at concentrations that are usually observed in arc-evaporation coatings, the value of F decreases linearly, proportional to the droplet concentration and almost independent of droplet thermal conductivity for regularly experimentally encountered droplet materials. When ellipsoidal droplets are considered, F depends strongly on the material and orientation of the droplets. The effects of finite thermal interface resistance between individual layers and around the droplet inclusions are evaluated separately and are found to be generally beneficial. This study shows that creating multilayers with high anisotropy of thermal conductivity should be possible even in the presence of unavoidable droplet inclusions. Furthermore, controlling metal droplet formation upon arc-evaporation of hard coatings can be used as a tool to engineer the anisotropy of thermal conductivity in arc-deposited multilayer coatings.

Highly porous carbon-fiber materials are used in ablative heat shields for atmospheric re-entry. These carbon fiber based materials perform well against heat convection and conduction, and have high radiation absorption. During re-entry, spacecrafts are exposed to shock layer radiation, which provides a considerable share of the heat load faced by their shields. In this work, models for reflectance and heat transfer through FiberForm, a porous carbon-fiber material, are described. The models are based on the multiphase approach and were validated by comparison with experimental data on reflectance and on effective high-temperature thermal conductivity. The morphology of FiberForm was reconstructed from X-ray tomography data and was used as an input for the models. Transmission electron microscopy and diffraction analyses of FiberForm revealed the presence of an amorphous phase and graphite nanocrystalline grains smaller than 10 nm, with no preferential crystallographic orientation. A strong influence of anisotropy in fiber orientation was observed in measured reflectance and has been included in modeling. The porosity and the extinction coefficient were obtained from X-ray tomography images. The radiative transfer modeling indicates that 90% of the incident radiation at wavelengths around 1 μm is absorbed in the 200–300 μm thick outermost surface layer.

Hardness, oxidation resistance and adhesion have traditionally been the most important parameters for material selection and optimization of hard, wear resistant coatings. More recently, thermal conductivity has become a focus of research, as it has the potential to have a significant effect on coating performance. Even though new experimental methods have become available to characterize thermal conductivity, recent studies focused on existing material systems, and there have been few attempts to design coating thermal conductivity for specific applications. We study thermal conductivity of arc evaporation oxynitride coatings with the composition CrN1 − xOx and TiN1 − xOx (x = 0 … 0.4). With increasing substitution of nitrogen by oxygen, thermal conductivity could gradually be reduced from 9 W m− 1 K− 1 to 2.5 W m− 1 K− 1 in CrN1 − xOx and from 35 to 5 W m− 1 K− 1 in TiN1 − xOx. A model that assumes a constant phonon scattering cross-section of the oxygen concentration can explain the observed functional relationship. Other properties such as thermal stability, oxidation resistance and hardness of these oxynitrides show little variation with oxygen concentration, clearing the way to design functional coatings with specific thermal properties.

A hybrid approach to flnd the optical response of periodic photonic structures to incident light is presented. The approach is based on a scattering matrix combination of the Finite Element Method (FEM) and the Fourier Modal Method (FMM). Optical response calculations include: scattering in both re∞ection and transmission directions, absorption and electric and magnetic fleld distributions inside the structure. The approach is tested on a structure | composed of dielectric and metallic materials | that is periodic in one direction. An analysis of the calculation accuracy shows that the approach depends on the subdivision into FEM and FMM domains and that the optimal subdivision depends on the calculations frequency range as well as on the structure geometry. For testing, we use the commercial FEM solver contained in CST Microwave Studio and a based on C/C++ Fourier Modal Method implementation.

The long-term stability of Ta 0.16 Y 0.16 Zr 0.68 O 2 (TaYSZ) has been studied for possible application in thermal barrier coatings. X-ray diffraction wasused for the characterization of the phase stability from 1100 to 1500 ◦ C. Long-term stability of TaYSZ in presence of 7YSZ at 1250 ◦ C has alsobeen checked. At 1500 ◦ C, TaYSZ remains as a single tetragonal phase. TaYSZ, however, when treated at 1250 ◦ C for 600h decomposes to m -ZrO 2 and contains a minor YTaO 4 phase and t-TaYSZ. In the presence of 7YSZ, decomposition of TaYSZ was suppressed. Decomposition of TaYSZfollows a different mechanism when treated in air and under vacuum. TaYSZ once pretreated at 1500 ◦ C does not show any decomposition whentreated at 1250 ◦ C for 600h, though weak reflections of Y 3 TaO 7 are seen in the X-ray diffraction pattern. The onset of sintering and the coefficientof thermal expansion (CTE) of TaYSZ have been found to be ∼1200 ◦ C and 11.24×10 −6 K −1 , respectively.© 2011 Elsevier Ltd. All rights reserved.

Some aspects of stable and equilibrium phases in the Ta 2 O 5 –Y 2 O 3 –ZrO 2 system have been studied in this work. Phase stability of Ta 0.16 Y 0.16 Zr 0.68 O 2 and Ta 0.10 Y 0.10 Zr 0.80 O 2 has been studied by X-ray powder diffraction. A secondary phase segregation of YTaO 4 has been observed in both compositions below 1500 °C. However, once treated at 1500 °C, YTaO 4 phase disappears. The preliminary phase diagram of the Ta 2 O 5 –Y 2 O 3 –ZrO 2 system at 1200 °C and 1500 °C have been constructed using THERMOCALC package. Thermodynamic description is obtained by critically evaluating experimental data on the binary systems Y 2 O 3 –Ta 2 O 5 and Ta 2 O 5 –ZrO 2 . Binary system data have been combined and parameters for ternary tetragonal and cubic structures have been assessed to construct ternary phase diagram. Calculated isothermal section at 1200 °C is similar to 1500 °C. There is no change in the tetragonal solid solution region observed with temperature, which indicates the same equilibrium phases at 1500 °C and at 1200 °C.

We present the design of one-dimensional photonic crystal structures, which can be used as omnidirectional reflecting shields against radiative heating of space vehicles entering the Earth’s atmosphere. This radiation is approximated by two broad bands centered at visible and nearinfrared energies. We applied two approaches to find structures with the best omnidirectional reflecting performance. The first approach is based on a band gap analysis and leads to structures composed of stacked Bragg mirrors. In the second approach, we optimize the structure using an evolutionary strategy. The suggested structures are compared with a simple design of two stacked Bragg mirrors. Choice of the constituent materials for the layers as well as the influence of interlayer diffusion at high temperatures are discussed.

The thermal conductivity of non-metallic nano- and microstructured materials is a key parameter when considering wear resistant coatings. Here, different models of phonon transport and estimation of thermal conductivity are analyzed. The hopping mechanism of phonon transport was found to be applicable for small-grain-size materials in understanding thermal conductivity, whereas large grain size materials can be studied using the correlation function approach. The Chapman–Enskog approach to the Boltzmann transport equation assuming that the transport of phonons is controlled by scattering on the grain boundaries has been analyzed. The Fourier law of thermal conductivity is obtained with the thermal conductivity inversely proportional to the specific surface of the boundaries.

Cr–O–N coatings were produced by reactive cathodic arc deposition at different N2/O2 flow ratios onto cemented carbide substrates. The structure, and mechanical properties of the coatings depend strongly on their oxygen content. The increase of the oxygen content leads to a decrease of the mean crystallite size of the nanocrystalline cubic (B1 structure) Cr–O–N phase and to an enhancement of the (002) preferred orientation. At O/(O + N) ratios > 0.7 the coatings crystallize in the rhombohedral Cr2O3 structure. The morphology of the samples, as studied by scanning electron microscopy (SEM), is columnar. The hardness of the coatings increases up to a maximal value of 28 GPa with increasing the oxygen content. Higher oxygen contents lower the hardness of the coatings. X-ray powder diffraction (XRD) studies were performed in situ at high temperatures, in vacuum and in air. The crystallite growth at elevated temperatures, both in vacuum and in air, is hindered significantly by the presence of oxygen in the coatings. The Cr–O–N coatings with the B1 structure, annealed in vacuum and in air, provide an improved thermal stability, with no evidence of oxidation or formation of the Cr2N phase up to 900 °C.

This work is focused on synthesis, characterization, and determination of main parameters of the multilayer P123 templated TiO2 films. The mesoporous multilayer thin films consist of TiO2 nanoparticles on the F-doped SnO2 (FTO) conductive glass substrates. The films were grown by implementing the protocol of supramolecular templating with the amphiphilic triblock copolymer, Pluronic P123. The templated multilayer films were manufactured by repeated dip coating followed by the thermal treatment at 350 °C for 2 h after deposition of each layer. It was found that the multilayer preparation technique at 350 °C has serious limitations. The structure does not further increase its specific surface area (roughness factor) after deposition of more than 3−5 layers. The new surface area added by deposition of the top layer is compensated by the reduction of the surface area lost due to the sintering of the bottom layers. The careful review of the analytical data suggests that the morphology of the P123 templated TiO...

Composition, structure and morphology of Cr–Si–N coatings deposited by cathodic arc ion-plating on cemented carbide substrates were studied. A systematic variation of the deposition parameters resulted in relative Si atomic concentrations Si/(Cr + Si) within 0 and 15at.%, which affect significantly the properties of the coatings, their phase stability and oxidation resistance. All Cr–Si–N coatings consist of nanocrystalline CrN grains with diminishing crystallite sizes at increased Si content. The microstructure of the samples, observed by scanning electron microscopy, is columnar for the coatings with low Si concentration, and becomes denser for specimens with higher Si content. The hardness and Young's modulus of the coatings increase with increasing Si concentration up to a maximum value of 26GPa and 430GPa, respectively, for the Cr 0.94 Si 0.06 N coatings, followed by a progressive decrease. X-ray powder diffraction studies were performed in situ up to 1000°C in vacuum and in air. Annealing in vacuum resulted in the decomposition of CrN into Cr 2 N and N 2 and the subsequent oxidation. The annealing performed in air proved an excellent oxidation resistance of the coatings, which strongly depends on their composition and morphology. The best oxidation resistance was obtained for the hardest samples with intermediate Si concentration, for which the formation of Cr 2 O 3 has not been observed even at 1000°C. Recrystallization, which occurs at elevated temperatures both in vacuum and in the air, becomes significant above approximately 800°C.

One failure mechanism of thermal barrier coatings composed of yttria-stabilized zirconia (YSZ) has been proposed to be caused, in part, by the transformation of the tetragonal phase of YSZ into its monoclinic phase. Normally, studies of phase evolution are performed by X-ray diffraction (XRD) and by evaluating the intensities of a few diffraction peaks for each phase. However, this method misses some important information that can be obtained with the Rietveld method. Using Rietveld's refinement of XRD patterns, we observed, upon annealing of YSZ coatings, an increase of cubic phase content, a reduction in as-deposited tetragonal phase content, and the appearance of a new tetragonal phase having a lower yttria content that coexists with the as-deposited tetragonal phase of YSZ.

The main directions of studies of materials with negative index of refraction, also called left-handed or metamaterials, are reviewed. First, the physics of the phenomenon of negative refraction and the history of this scientific branch are outlined. Then recent results of studies of photonic crystals that exhibit negative refraction are discussed. In the third part numerical methods for the simulation of negative index material configurations and of metamaterials that exhibit negative index properties are presented. The advantages and the shortages of existing computer packages are analyzed. Finally, details of the fabrication of different kinds of metamaterials are given. This includes composite metamaterials, photonic crystals, and transmission line metamaterials for different wavelengths namely radio frequencies, microwaves, terahertz, infrared, and visible light. Furthermore, some examples of practical applications of metamaterials are presented.

Thin colloidal photonic crystals were grown using shear-flow crystallization of polystyrene colloidal nanoparticles. The arrays consist of two types of domains, which are clearly visible on scanning electron microscopy patterns. Both domains have cubic close packing and are distinguished by their orientation with regards to substrate surface. The positions of the main minima in the visible-near-IR transmittance spectrum of the arrays as well as the dependence of these minima on the angle of the light incidence have been measured and interpreted using photon band structure of the two-component media.

The thermophysical properties of B2 structure based Al–Ni–Ru–M alloys have been studied for different alloy compositions. A normal value for the thermal conductivity of these alloys is within the range of 10–20 W m −1 K −1 range at room temperature, but can be reduced to approximately 3.5 W m −1 K −1 by modifying the alloy composition. The observed positive and approximately linear temperature dependence of the thermal conductivity of B2 structure based on Al–Ni–Ru–M alloys indicates their metal electronic nature. Thermal expansion coefficients (i) increase with temperature, (ii) are similar to those of superalloys and steels, and (iii) are mostly dependent on Ru content. A fused and subsequently pulverized Al–Ni–Ru B2 alloy was deposited on a Ni-based superalloy substrate made of modified CMSX-4 using vacuum and atmospheric plasma spraying. The coatings obtained were characterized for their microstructure, phase composition, hardness, and bond strength. The coatings exhibited sound microstructural integrity and favorable coating–substrate adhesion. The phases originally present in the powder were also found to be retained in the coating. Interdiffusion processes between the investigated coatings and the Ni-based superalloy substrate were primarily estimated by the diffusion couples approach. The segregation of a new intermetallic phase was detected at the (Al 50 Ni 40 Ru 10 alloy)/(modified CMSX-4) interface.

During the past five years, the authors have developed in their laboratory a new type of solar cell that is based on a photoelectrochemical process. The light absorption is performed by a monolayer of dye (i.e., a Ruthenium complex) that is adsorbed chemically at the surface of a semiconductor (i.e., titanium oxide (TiO{sub 2})). When excited by a photon, the dye has the ability to transfer an electron to the semiconductor. The electric field that is inside the material allows extraction of the electron, and the positive charge is transferred from the dye to a redox mediator that is present in solution. A respectable photovoltaic efficiency (i.e., 10%) is obtained by the use of mesoporous, nanostructured films of anatase particles. The authors show how the TiO{sub 2} electrode microstructure influences the photovoltaic response of the cell. More specifically, they focus on how processing parameters such as precursor chemistry, temperature for hydrothermal growth, binder addition, and sintering conditions influence the film porosity, pore-size distribution, light scattering, and electron percolation and consequently affect the solar-cell efficiency.

Phase states and oxidation resistance of B2-structure based Al–Ni–Ru–M alloys have been investigated on equilibrium alloys and by the diffusion couple approach. The isothermal cross-sections of the Al–Ni–Ru phase diagram have been partially defined in a range of Al25 at.%. The Ru layer can contain considerable amounts of Cr or Ir and exhibits good adhesion to both the scale and the bulk alloy. Physical properties of the B2 structure based Al–Ni–Ru–M alloys make them good candidates for oxidation resistant coatings in different high temperature industrial applications.

The adsorption of formate and acetate from aqueous solutions at pH 3−9 onto the TiO2 rutile (110) surface was studied by ATR-FTIR spectroscopy. The spectra indicated that there was only one type of adsorbed species, and that formate and acetate were adsorbed in a similar manner. On the basis of the measured νas(COO) − νs(COO) splitting (Δνas-s) of 191 and 87 cm-1, for formate and acetate, respectively, the monodentate (ester type) binding mode could be excluded. Ab initio calculations at the Hartree−Fock level showed that for pentacoordinated TiIV, present in the (110) surface, the chelating bidentate binding mode is unstable with respect to the rearrangement to the monodentate or the bridging bidentate mode. The computed vibrational frequencies of formate and acetate adsorbed in a bridging bidentate mode onto Ti clusters with 2−5 Ti centers, representing the (110) surface, agreed with experiment and thus showed that this methodology can be used for the determination of the structures of adsorbates on, fo...

Simple one-dimensional quantum models of electron injection from excited photosensitizer molecules to semiconductor nanoparticles and of further electron motion through the TiO 2 nanoparticle are considered. It was found that essential parameters of the models may correspond well to the real values of spatial and energetic parameters of ruthenium sensitizer complexes absorbed on the TiO 2 surface.

Reference LPI-ARTICLE-2002-013doi:10.1002/aoc.361View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

Prototype solar cells made up of nanocrystalline anatase supported on a conducting glass and sensitized with two Ru2+ complexes (cis-dithiocyanatobis(2,2‘-bipyridine-4,4‘-dicarboxylate)ruthenium(II), or red dye, and (4,4‘,4‘ ‘-tricarboxy-2,2‘:6‘,2‘ ‘-terpyridine)ruthenium(II), or black dye) are characterized with out-of-plane X-ray diffraction, X-ray photoelectron spectroscopy, and Ti K-edge XAFS (XANES and EXAFS), as well as by powder X-ray diffraction of the pure dyes used for a coating of a TiO2 layer. The data indicate, within the sensitivity limits of the methods applied, that the dyes most probably form (partial coverage) a monolayer at the surface of anatase nanoparticles. Coating with the dyes leads to a stronger distortion of the local environment of Ti atoms in anatase nanoparticles.

A double-cell spectrum splitting scheme for solar photovoltaics is investigated. A simple and flexible design promises both terrestrial and space applications at low costs. The investigation was performed using the Fourier Modal Method based on the scattering matrix formalism. A numerical optimization was done by an evolutionary strategy algorithm. The optimized design is predicted to have over 41% solar conversion efficiency limit cells without sun light concentration. We believe that the proposed design has the potential of becoming a cost-efficient photovoltaic technology.

A new series of panchromatic ruthenium(II) sensitizers derived from carboxylated terpyridyl complexes of tris-thiocyanato Ru(II) have been developed. Black dye containing different degrees of protonation [(C(2)H(5))(3)NH][Ru(H(3)tcterpy)(NCS)(3)] 1, [(C(4)H(9))(4)N](2)[Ru(H(2)tcterpy)(NCS)(3)] 2, [(C(4)H(9))(4)N](3)[Ru(Htcterpy)(NCS)(3)] 3, and [(C(4)H(9))(4)N](4)[Ru(tcterpy)(NCS)(3)] 4 (tcterpy = 4,4',4' '-tricarboxy-2,2':6',2' '-terpyridine) have been synthesized and fully characterized by UV-vis, emission, IR, Raman, NMR, cyclic voltammetry, and X-ray diffraction studies. The crystal structure of complex 2 confirms the presence of a Ru(II)N6 central core derived from the terpyridine ligand and three N-bonded thiocyanates. Intermolecular H-bonding between carboxylates on neighboring terpyridines gives rise to 2-D H-bonded arrays. The absorption and emission maxima of the black dye show a bathochromic shift with decreasing pH and exhibit pH-dependent excited-state lifetimes. The red-shift of the emission maxima is due to better pi-acceptor properties of the acid form that lowers the energy of the CT excited state. The low-energy metal-to-ligand charge-transfer absorption band showed marked solvatochromism due to the presence of thiocyanate ligands. The Ru(II)/(III) oxidation potential of the black dye and the ligand-based reduction potential shifted cathodically with decreasing number of protons and showed more reversible character. The adsorption of complex 3 from methoxyacetonitrile solution onto transparent TiO(2) films was interpreted by a Langmuir isotherm yielding an adsorption equilibrium constant, K(ads), of (1.0 +/- 0.3) x 10(5) M(-1). The amount of dye adsorbed at monolayer saturation was (n(alpha) = 6.9 +/- 0.3) x 10(-)(8) mol/mg of TiO(2), which is around 30% less than that of the cis-di(thiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) complex. The black dye, when anchored to nanocrystalline TiO(2) films achieves very efficient sensitization over the whole visible range extending into the near-IR region up to 920 nm, yielding over 80% incident photon-to-current efficiencies (IPCE). Solar cells containing the black dye were subjected to analysis by a photovoltaic calibration laboratory (NREL, U.S.A.) to determine their solar-to-electric conversion efficiency under standard AM 1.5 sunlight. A short circuit photocurrent density obtained was 20.5 mA/cm(2), and the open circuit voltage was 0.72 V corresponding to an overall conversion efficiency of 10.4%.

Thin layer electrodes (0.2−0.5 μm) of highly organized nanotextured anatase were prepared by hydrolysis of TiCl4 in the presence of poly(alkylene oxide) block copolymer, Pluronic P-123, acting as the structure-directing agent. Electrochemical properties of these layers were studied in KCF3SO3 + propylene carbonate and in LiN(CF3SO2)2 + ethylene carbonate + dimethoxyethane. The electrodes showed unusually fast capacitive and Li-insertion charging. A most striking effect was the occurrence of two new pairs of peaks in cyclic voltammograms in Li+ containing electrolyte solutions. These, so-called S-peaks, appear in addition to the “ordinary” peaks of Li insertion into anatase. The S-peaks can act as indicators of mesoscopic ordering of the skeleton as they diminish after mechanical or thermal destruction of the organized nanotexture. We suggest that the occurrence of S-peaks is connected to the presence of amorphous TiO2 in the organized skeleton.

Theoretical analyies of light absorption in the microcrystalline transition-metal oxide semiconductors is proposed. The interface mechanism of light absorption, where the indirect interband electron transitions are caused by the possibility of the momentum nonconservation at the interface, is considered. This mechanism is found to be most important for crystallites, the size of which is about a few 10 nm or less.

Nanocrystalline Nb2O5 films were prepared by an extended sol-gel method. The synthesis is based on the hydrolysis of a modified Nb-alkoxide precursor. Reaction of the modified precursor (Nb(OEt)5 + 2 2,4-pentanedione) with water in ethanol leads to a homogeneous hydrolyzed solution, which is stable against precipitation of niobium oxide after evaporation of the ethanol and in the whole pH-range investigated (1–10). Autoclaving leads to amorphous gels, from which homogeneous nanocrystalline niobium oxide films of up to 15 μm can be made. During annealing crystalline phases are first observed above 500°C with fully crystalline films of orthorhombic T-phase Nb2O5 attained at 600°C. The microstructural, crystallographic, optical and photoelectrical properties of the films were characterized by means of SEM, XRD, UV-VIS spectroscopy and surface photovoltage spectroscopy, respectively.

Three-layer structures of thick (>5 μm) films of nanosized titanium dioxide, zirconium dioxide, and carbon have been screen-printed on a semi-production level for use in photosensitized photovoltaic devices. The films have been characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and nitrogen adsorption/desorption. The rheology of the screen-printing pastes has been measured, and qualitatively matched to the quality of the resultant film. Three titanium dioxide candidates were evaluated for performance. At low light levels (

Light absorption in TiO2 nanoparticles is analyzed. The interface mechanism of light absorption, where the indirect interband electron transitions are caused by the possibility of the momentum nonconservation at the interface, is considered. This mechanism is found to be most important for crystallites, which are generally a few 10 nm or less in size.

Kinetics of variations in the components of total impedance for single-crystal Pb0.75Sn0.25Te:In samples is investigated upon switching on and switching off of the illumination source at temperature 4.2 K and frequency 106 Hz. The calculation carried out in the context of representation of equivalent circuits showed that the parameters of a parallel R p C p chain, which approximates the sample behavior, correlate strictly. In the region of low resistances R p , the values of C p increase abruptly following the law C p ∝ R −2 . It is of no importance what method of decreasing the resistance is used—heating of the sample or its illumination at low temperature. The obtained results were analyzed taking into account the contribution of the impurity subsystem to conductivity and the possible manifestation of Maxwell-Wagner-type effects.

A general method for the synthesis of tris-heteroleptic ruthenium(II) complexes of the type [Ru(dcbpy)(dmbpy)(ddtc)] (3) and [Ru(dcbpy)(dmbpy)(NCS)2] (4) is reported, where the ligands (dcbpy = 4,4‘-dicarboxy-2,2‘-bipyridine, dmbpy = 4,4‘-dimethyl-2,2‘-bipyridine, and ddtc = diethyldithiocarbamate) are introduced sequentially. The complexes have been characterized by UV/visible, emission, IR, Raman and NMR spectroscopies and cyclic voltammetry. The effect of pH on the absorption spectra and luminescence behavior of these complexes consisting of protonatable ligands has been investigated in a water/ethanol solvent mixture by pH titration. The dissociation of protons is in sequential steps (pKa = 3.5 and 1.8). The excited-state pKa values are more basic than the ground-state pKa values. Resonance Raman spectra of these complexes strongly suggest that the lowest-energy metal-to-ligand charge-transfer transition bands are localized on the dcbpy ligand. The molecular crystal structure of 3 and the performance ...

Nanocrystalline titanium dioxide colloids have been synthesized using a sol−gel technique followed by growth under hydrothermal conditions in a basic environment at temperatures between 190 and 270 °C. Thin films have been made from aqueous suspensions of these colloids. X-ray analysis showed the colloids to be primarily the anatase crystal phase. Scanning electron microscopy (SEM) revealed a predominantly rodlike particle morphology after growth at lower temperatures and the formation of principally truncated tetragonal or tetrahydral bipyramidal nanocrystallites following growth at higher temperatures. The rodlike particles self-organize into regular cubic arrays with the long axis of the rods aligned perpendicular to the film surface. This self-organization is dependent upon the base used in colloidal synthesis and also upon the dielectric constant of the medium used during film formation.

Reference LPI-ARTICLE-1998-002doi:10.1016/S0020-1693(97)05989-6View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

The influence of the sharpness of the interface on the electron transport is investigated. The simple one-dimensional tight-binding model, which allows the consideration of an interface with arbitrary sharpness, is examined. It is shown that an interface with size much larger than a( λ a ) 1 3 (where a is the lattice constant and λ is the electron wavelength) can be considered as smooth. This means that the interface transmittance for the electron crossing is of the order of unity. In the opposite limiting case of the sharp interface, the transmittance becomes suppressed as ( a λ ) 2 .

Reference LPI-ARTICLE-1997-034doi:10.1006/jssc.1997.7409View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

The effect of the roughening of the silver surface upon photoemission observed in the presence of two different scavengers dissolved in an aqueous solution, CO2 and NO3- ions, was re-examined. The quantum yield of the photocurrent exhibits a sharp maximum at 370−380 nm in the frequency range of surface plasmons on silver. These photoyields were strongly affected by the extent of the roughening of the Ag surface and reached an unusually large value of 0.05 electron per incident photon. Electrochemical oxidation/reduction roughening performed in the presence of Cl-, Br-, and ClO4- ions, similar to that employed in enhanced Raman scattering (SERS) experiments, affected the maximum of the photocurrent and in some cases also its onset potential. Photocurrents arising at potentials more negative than the potential of zero charge (pzc) of silver were principally influenced by changing morphology of the surface resulting from more or less deep roughening performed in different solutions. On the other hand, the ph...

Scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and X-ray powder diffraction (XRD) studies on nanocrystalline TiO2 powders and thin films are presented. The size, shape (mostly exposed faces), and ordering of the TiO2 anatase particles in the nanocrystalline films are discussed. The use of the topochemical approach, which considers the properties of (nanocrystalline) solids in terms of crystallographic features of (nano)crystals is suggested. The surface area of sensitizer [bis(4,4‘-dicarboxy-2,2‘-bipyridine)bis(thiocyanato)]ruthenium(II) [abbreviated as (cis-Ru(dcbpy)2(NCS)2] on the semiconductor surface for the different types of anchoring is estimated on the basis of single-crystal X-ray diffraction studies of the esterified form of the complex.

The crystal study of tris(4,4′-dimethoxy-2,2′-bipyridine)iron(II)bis(hexafluorophosphate) (crystal solvate with N -methyl-2-pyrrolidone) was performed at 295 K. The crystals are triclinic, a = 11.721(4), b = 12.864(6), c =15.664(7) A , α = 77.14(4),β = 85.03 (3), γ = 81.76(3)° , space group P 1 , Z = 2, R = 0.0679 for 4694 reflections. The average FeN distances of 1.967 A agrees well with those observed in other tris(2,2-bipyridine) complexes of hexacoordinated Fe(II) with a large low-spin fraction. A small difference of the individual FeN distances in the dication (which are shorter for one of the bipyridine ligands) may be explained by crystal packing effects. A very specific orientation of dications [Fe(DMO-bpy) 3 ] 2+ in the crystal is worthy of note (the vector of one of the moieties N…(Fe)…N of the dication coincides in fact with direction [011] of the crystal). The possible interaction of the trication of tris(4,4′-dimethoxy-2,2′-bipyridine)iron (III) with the active site of glucose oxidase (i.e. the metal complex acting as an oxidant/electron-transfer mediator) is discussed.

Chemical (by interaction with a dibutylmagnesium solution) and electrochemical (in acetonitrile solution of magnesium perchlorate) insertion of Mg2+into the single crystals of V2O5was performed. The morphology change of V2O5crystals as a result of the Mg2+insertion was studied by scanning electron microscopy. The wavelength dispersive electron probe microanalysis clearly showed the presence of Mg (at least) at the surface of intercalated V2O5. Based on the single crystal X-ray diffraction study of intercalated V2O5(orthorhombic,Pmn21,a= 11.544(6),b= 4.383(3),c= 3.574(2) A,Z= 4) the location of a small amount of Mg (∼1%) in the bulk V2O5may be suggested, with [6 + 4] oxygen atoms surrounding Mg. The resulting Mg–O separations essentially exceed the accepted values for the Mg–O distances in crystals with hexacoordinated Mg atoms, which may be correlated with the structural and electrochemical properties of Mg2+-inserted V2O5.