Minimal Layer Thickness Research Articles

Fundamental absorption bandwidth to thickness limit for transparent homogeneous layers

Abstract Past work has considered the analytic properties of the reflection coefficient for a metal-backed slab. The primary result established a fundamental relationship for the minimal layer thickness to bandwidth ratio achievable for an absorber. There has yet to be establishment of a similar relationship for non-metal-backed layers, and here we present the universal result based on the Kramers–Kronig relations. Our theory is validated with transfer matrix calculations of homogeneous materials, and full-wave numerical simulations of electromagnetic metamaterials. Our results place more general fundamental limits on absorbers and thus will be important for both fundamental and applied studies.

Curing parameters to improve the mechanical properties of stereolithographic printed specimens

PurposeThis paper presents findings from a study examining curing procedures to improve the compressive strength and hardness properties of specimens while maintaining surface quality. All specimens were created from a standard grey, acrylic-based photopolymer and fabricated using stereolithography technology. This paper aims to investigate the effects of printing layer thickness and print orientation on specimen compressive strength, as well as the effects of thermal and light curing methods. In addition, the post-print curing depth was investigated.Design/methodology/approachThe effects of layer thickness and print orientation were investigated on 10 × 20 mm cylinders by determining the ultimate compressive strength once cured. The compressive strength of cylinders subjected to varying thermal and light settings was also investigated to determine the optimal curing settings. The effective depth of curing was investigated on a 25.4-mm cuboidal specimen, which received both thermal and light curing.FindingsTo achieve the highest compressive strength, specimens shall be printed with the minimal layer thickness of 25 µm. Increasing temperatures up to 60° C during curing provided a 0.75-MPa increase in compressive strength per degree Celsius. However, increasing temperatures above 60° C only provided a 0.15-MPa increase in compressive strength per degree Celsius. Furthermore, curing temperatures above 110° C resulted in degraded surface quality noted by defects at the layer laminations. Specimens required a minimum light curing exposure time of four hours to reach the maximum cure at which point any increase in exposure time provided no substantial increase in compressive strength.Originality/valueThis study provides recommendations for printing parameters and curing methods to achieve the optimum mechanical properties of cured stereolithography specimens.

Multi-layer topography measurement using a new hybrid single-shot technique: Chromatic Confocal Coherence Tomography (CCCT).

Often measurement tasks occur, where specimens consist of multiple layers or topography shall be examined through contaminations. Especially for unknown layer materials, it is important to measure the layer's refractive index to compensate for the errors induced on the measurement of underlying surfaces. Chromatic Confocal Coherence Tomography is proposed as a new hybrid single-shot scheme for a simultaneous measurement of thickness and refractive index of semitransparent layers, combining chromatic confocal and interferometric information. As a proof of concept, first measurements are presented along with a short discussion about their uncertainties, where minimal layer thickness and resolution are dominated by the confocal part of the signal, that is mainly influenced by the chosen microscope objective.

Transient convective structures in a cooled water layer in the presence of a drift flow and a surfactant

The paper is concerned with three-dimensional convective structures arising in a water layer cooled from above and covered by an adsorbed insoluble surfactant. The water is subjected to a laminar drift flow produced by tangential stresses on a free surface. The surface diffusion of the surfactant are taken into account within the approximation of a nondeformable flat surface. After appropriate reformulation of standard equations governing gravity-capillary convection and surfactant concentration, the problem is solved numerically using a pseudospectral method employed in our previous work. Development of the convective structures with increasing Reynolds number, surfactant film elasticity, and layer thickness is studied. The minimal layer thickness is chosen taking into account the results of relevant laboratory experiments. The cell-to-roll transition is revealed in the thin layer with increasing Reynolds number. The role of dissipation due to the surfactant film is elucidated by comparison with purely gravitational convection. The turbulent convection arising in a thicker layer subjected to a laminar shear flow is examined. Disordered streets containing elongated cells and swirl-like motions are revealed. Images of the surface temperature and the perturbed surfactant concentration are compared.

Assembly of 8YSZ nanoparticles into gas-tight 1–2 μm thick 8YSZ electrolyte layers using wet coating methods

The application of a thin film electrolyte layer with a thickness in the micrometer range could greatly improve current solid oxide fuel cells (SOFCs) in terms of operating temperature and power output. Since the achievable minimal layer thickness with conventional powder coating methods is limited to ∼5 μm, a variety of thin film methods have been studied, but results on regular large-scale anode substrates are still lacking in the literature. In this paper, a wet coating process is presented for fabricating gas-tight 1–2 μm thick 8YSZ electrolyte layers on a regular NiO/8YSZ substrate, with a rough surface, a high porosity and a large pore size. These layers were deposited in a similar way as conventional suspension based layers, but the essential difference includes the use of coating liquids (nano-dispersion, sol) with a considerably smaller particle size (85 nm, 60 nm, 35 nm, 6 nm). Successful deposition of such layers was accomplished by means of an innovative coating process, which involves the preparation of a hybrid polyvinyl alcohol/8YSZ membrane by dip-coating or spin-coating and subsequently burning out the polymer part at 500 °C. Results from He leak tests confirmed that the sintered layers posses a very low number of defects and with values in the range 10 −4–10 −6 (hPa dm 3)/(s cm 2) the gas-tightness of the thin film layers is satisfactory for fuel cell operation. Moreover, preliminary results have also indicated a potential reduction of the sintering temperature from 1400 °C to the range 1200–1300 °C, using the presented coating process.

Aperiodic normal-incidence antimony-based multilayer mirrors in the 8 — 13-nm spectral range

The optical properties of several materials were analysed from the standpoint of fabrication of broadband normal-incidence multilayer mirrors possessing maximal uniform reflectivity in the 8–13-nm range. By solving the inverse problem of multilayer optics we show that aperiodic Sb/(B4C, Sc, Si) multilayer structures optimised for maximum uniform reflectivity in the 8–13-nm range are able to afford a normal-incidence reflectivity R∼10 % throughout this range. The best results are exhibited by the pair Sb/B4C, for which the average reflection coefficient amounts to about 13%. The dependence of optimisation result on the programmable limitation on the minimal layer thickness in the multilayer structure was numerically investigated. An empirical rule was established whereby setting the lower bound for a layer thickness at a level ∼λmin/4 (in this case, λmin = 8 nm) does not result in an appreciable lowering of attainable uniform reflectivity.

Key techniques of selective laser melting rapid prototyping system

The key points of the three main subsystems of se- lective laser melting(SLM)rapid prototyping system such as the laser,the powder delivering and the scanning systems,are studied through calculation of the needed laser energy during SLM processing.The selection of laser to be used is confirmed. It is found that the accuracy of the cylinder movement,the ec- centricity and interval in the radial run-out of the roller have important influences on the control of minimal layer thickness. Combined with an experiment of selective melting of metal powder at varying scanning speeds,the scanning performance is analyzed,and the result shows that fast scanning speed and proper scanning strategy should be used during SLM processing to avoid thermal deformation.Based on above research work, an experiment for producing metal part using Cu-based powder is carried out.The result shows the part is metallurgical bonded entity with density of 95%,dimension precision of±0.5 mm, and the microstructure of the part is composed of equiax and dendritic crystal.The cross section and the shape of the SLM part,the processing parameters such as layer thickness and scanning speed,demonstrate the key points of SLM technique.

Alloy Synthesis by Atomic Layer Laminations for Read Sensor Applications

ABSTRACTA novel method has been developed for synthesis of multi-element alloys by planetary magnetron sputter deposition techniques. The alloy is formed by alternating depositions of uniform ultra-thin component layers at a fraction of a mono-atomic layer, or atomic layer lamination. A minimal layer thickness of down to 0.1Å can be obtained such that the components can be mixed at atomic level forming a homogeneous alloy.A typical example is demonstrated here for the synthesis of CoxFe1−x alloys by alternating depositions of Co and Fe layers from pure element targets. The compositional dependence of the laminates basically reconstructs the well-known Slater-Pauling curve of the bulk CoxFe1−x alloys. The laminates exhibit a clear structural transition at Co compositions between 60 and 80 % from fcc phase to bcc phase, characteristic of bulk CoxFe1−x alloys. The novel atomic layer lamination has been successfully applied to the composition optimization of CoFe pinned layer with a best pinning strength at Co70Fe30 for exchange biased spin-valve films.

SIMS analysis of AlxGa1–xas/gaas layered structures grown by metal‐organic vapour phase Epitaxy

AbstractFor AlxGa1‐xAs/GaAs layered structures, grown by MOVPE, abrupt interfaces in the order of one atomic distance were observed. These structures are well suited to serve as test samples for studying the influence of bombardment conditions on the quality of SIMS depth profiles. The results of such a study are presented and it is shown that depth profiles can be obtained with a depth resolution of about 25 Å. It was found that beyond the interface secondary ion signals decrease exponentially; from the variation of the slope of this exponential decay as a function of primary ion energy it can be concluded that depth resolution is determined by a purely collisional mixing process. From this type of data we can derive a minimal layer thickness in AlxGa1‐xAs/GaAs quantum well structures for which quantitative SIMS analysis of the Al content is possible.

Optimal distribution of density and dilatation modulus in inhomogeneous layers

The extremals of the minimal layer thickness problem for reflectance modification, given monochromatic waves at normal incidence and real material parameters, are analyzed. The structures of sublayer sequences leading to minimal total layer thickness, when both the density and the dilatation modulus are allowed to vary independently are presented. It is found that there exist two different sublayer sequences. Both are periodic with unit cells of four layers. Minimal sequences for the corresponding one-parameter problems are obtained as limiting cases.