Leonid Braginsky

Hard wear-resistant coatings with anisotropic thermal conductivity for high thermal load applications

High thermal load applications such as high speed dry cutting lead to high temperatures in the coated tool substrate that can soften the tool and high temperature gradients that can put stress on the coating/tool interface. In this work, theoretical considerations are presented for multilayer and graded protective coatings that can induce a significant anisotropy in their thermal conductivity. Solution of the heat equation shows that anisotropy of thermal conductivity has the potential to reduce temperatures and temperature gradients arising due to brief, localized heat at the coating surface (“hot-spots”). Experimental realization of high anisotropy is demonstrated in TiN/AlCrN multilayer coatings with variable layer thickness. In the coating with 50 nm bilayer periodicity, the highest anisotropy was obtained with a value of κ||/κ⊥=3.0±0.9. Time-domain thermoreflectance is used to measure in-plane and cross-plane thermal conductivity of fabricated coatings. The observed high values of anisotropy of therm...

Potential of Glassy Carbon and Silicon Carbide Photonic Structures as Electromagnetic Radiation Shields for Atmospheric Re-entry

During high-velocity atmospheric entries, space vehicles can be exposed to strong electromagnetic radiation from ionized gas in the shock layer. Glassy carbon (GC) and silicon carbide (SiC) are candidate thermal protection materials due to their high melting point and also their good thermal and mechanical properties. Based on data from shock tube experiments, a significant fraction of radiation at hypersonic entry conditions is in the frequency range from 215 to 415 THz. We propose and analyze SiC and GC photonic structures to increase the reflection of radiation in that range. For this purpose, we performed numerical optimizations of various structures using an evolutionary strategy. Among the considered structures are layered, porous, woodpile, inverse opal and guided-mode resonance structures. In order to estimate the impact of fabrication inaccuracies, the sensitivity of the reflectivity to structural imperfections is analyzed. We estimate that the reflectivity of GC photonic structures is limited to 38% in the aforementioned range, due to material absorption. However, GC material can be effective for photonic reflection of individual, strong spectral line. SiC on the other hand can be used to design a good reflector for the entire frequency range.

Ultrasensitive switching between resonant reflection and absorption in periodic gratings

Guided-Mode Resonance (GMR) efiects in transparent periodic gratings possess a number of remarkable phenomena. GMRs exhibit strong features in the optical spectrum, i.e., dips, peaks, cusps, and may attain extremely high Q-factors. In some cases, resonant re∞ection with the e-ciency equal to unity can be observed. We demonstrate that the introduction of small losses in the structure can drastically modify its optical response by causing strong absorption resonances. Unity re∞ection in loss-free structures can be almost completely converted into unity absorption peaks as soon as very small losses are introduced. Even thin absorbing fllms in the structure (or in its vicinity) can lead to such strong resonant absorption efiects. The resonances may exhibit a negligible spectral shift, but a signiflcant variation in the magnitude when losses are slightly altered, which is highly attractive for sensor and switch applications. Absorption peaks experience a resonant behavior with respect to both frequency and material losses. We show that the width of the absorption peaks decreases and approaches the width of the re∞ection peaks, as losses decrease. Thus, high-Q resonances can be observed. The absorption resonances also possess strong angular dependence; they may split and signiflcantly increase in magnitude for a slightly inclined incidence. We elucidate the resonant re∞ection/absorption efiects theoretically and provide numerical examples.

Fast numerical methods for the design of layered photonic structures with rough interfaces

A multilayer approach (MA) and modified boundary conditions (MBC) are proposed as fast and efficient numerical methods for the design of 1D photonic structures with rough interfaces. These methods are applicable for the structures, composed of materials with an arbitrary permittivity tensor. MA and MBC are numerically validated on different types of interface roughness and permittivities of the constituent materials. The proposed methods can be combined with the 4x4 scattering matrix method as a field solver and an evolutionary strategy as an optimizer. The resulted optimization procedure is fast, accurate, numerically stable and can be used to design structures for various applications.

Practical aspects of interaction of real photonic crystal with light

The envelope function approach for the electric and magnetic fields of the light wave in photonic crystals is proposed. We used this approach to investigate the light propagation in disordered photonic crystals and its reflection/refraction at the boubdary. We showed that small long-range distortion of the crystal lattice can explain pecularities of the translittance spectrum of photonic crystals.

Spectrum splitting double-cell scheme for solar photovoltaics

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.

Design of Reflective, Photonic Shields for Atmospheric Reentry

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.

Photonic band gap materials: fabrication, modeling, and applications

Methods of fabrication of the photonic band gap materials are reviewed, advantages of the combined use of templating and self-assembling methods are stressed. The envelope function approach was used to consider the light field in perfect photonic crystal as the zero-order approximation. Then distortion of the photonic structure has been introduced as perturbation. The simple model used in this paper allows consideration of the effects of short-range and long-range irregularities on transmission spectra. The examples of possible application of the suggested modeling approach and micro- and nanofabrication methods include the enhancement of target detectability, as possible functional elements in focal plane arrays of infrared detectors, controlled introduction of defects, prevention certain waves from propagating though or from photonic band gap materials, detection of direction to the light source, control of temperature distribution by thermal management of microstructure, negative permeability at visible frequencies.

Study of surfactant bonding to lead telluride nanoparticles

Colloidal suspensions of PbTe nanoparticles coated with oleic acid were obtained using a high-temperature solution phase method. Bonding between the oleic acid and nanoparticles was investigated using Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy. The results indicated that oleic acid coordinated the nanoparticles symmetrically through carboxyl groups, and the interaction modes were bridging bidentate and chelating interactions. The strong coordination by the capping monolayer made the nanoparticles stable against oxidation. Nanocrystalline PbTe exhibited electrical conductivity, which itself had an activation character with increasing temperature.

Radiation Heat Transfer in Porous Materials

The effect of porosity on the radiation component of the thermal conductivity of the thermal barrier coatings is studied. Heat transfer in the disordered porous structures as well as the porous photonic bandgap structures is investigated. The pores, which size is comparable with the characteristic radiation wavelength λmax=2897.8/T μm, were found to be most efficient obstacles for the heat radiation.