Nanoindentation in Crystal Engineering: Quantifying Mechanical Properties of Molecular Crystals

Nanoindentation is a technique for measuring the elastic modulus and hardness of small amounts of materials. This method, which has been used extensively for characterizing metallic and inorganic solids, is now being applied to organic and metalorganic crystals, and has also become relevant to the subject of crystal engineering, which is concerned with the design of molecular solids with desired properties and functions. Through nanoindentation it is possible to correlate molecular-level properties such as crystal packing, interaction characteristics, and the inherent anisotropy with micro/macroscopic events such as desolvation, domain coexistence, layer migration, polymorphism, and solid-state reactivity. Recent developments and exciting opportunities in this area are highlighted in this Minireview.

Experiments and zero D modeling studies using specific Wiebe coefficients for producer gas as fuel in spark-ignited engines

The paper addresses experiments and modeling studies on the use of producer gas, a bio-derived low energy content fuel in a spark-ignited engine. Producer gas, generated in situ, has thermo-physical properties different from those of fossil fuel(s). Experiments on naturally aspirated and turbo-charged engine operation and subsequent analysis of the cylinder pressure traces reveal significant differences in the heat release pattern within the cylinder compared with a typical fossil fuel. The heat release patterns for gasoline and producer gas compare well in the initial 50% but beyond this, producer gas combustion tends to be sluggish leading to an overall increase in the combustion duration. This is rather unexpected considering that producer gas with nearly 20% hydrogen has higher flame speeds than gasoline. The influence of hydrogen on the initial flame kernel development period and the combustion duration and hence on the overall heat release pattern is addressed. The significant deviations in the heat release profiles between conventional fuels and producer gas necessitates the estimation of producer gas-specific Wiebe coefficients. The experimental heat release profiles are used for estimating the Wiebe coefficients. Experimental evidence of lower fuel conversion efficiency based on the chemical and thermal analysis of the engine exhaust gas is used to arrive at the Wiebe coefficients. The efficiency factor a is found to be 2.4 while the shape factor m is estimated at 0.7 for 2% to 90% burn duration. The standard Wiebe coefficients for conventional fuels and fuel-specific coefficients for producer gas are used in a zero D model to predict the performance of a 6-cylinder gas engine under naturally aspirated and turbo-charged conditions. While simulation results with standard Wiebe coefficients result in excessive deviations from the experimental results, excellent match is observed when producer gas-specific coefficients are used. Predictions using the same coefficients on a 3-cylinder gas engine having different geometry and compression ratio(s) indicate close match with the experimental traces highlighting the versatility of the coefficients.

Mixing enhancement in a compact trapped vortex combustor

Previous studies on a single-cavity, compact trapped vortex combustor concept showed good flame stability for a wide range of flow conditions. However, achieving good mixing between cavity products and mainstream flow was still a major challenge. In the present study, a passive mixing enhancement strategy of using inclined struts along with a flow guide vane is presented and experimentally tested at atmospheric pressure conditions. Results show excellent mixing and consequently low values of the combustor exit pattern factor in the range of 0.1 and small flame lengths (57 times the main-duct depth). The pressure drop is small in the range of 0.35%, and NOx levels of the order of 12ppm are achieved. The flame stability is excellent, and combustion efficiency is reasonable in the range of 96%. The effectiveness of the proposed strategy is explained on the basis of in-situ OH chemiluminescence images and prior numerical simulations of the resulting complex flow field. The flow guide vane is observed to lead to a counterclockwise cavity vortex, which is conducive to the rise of cavity combustion products along the inclined struts and subsequent mixing with the mainstream flow.

Influence of physical structure and chemistry of diesel soot suspended in hexadecane on lubrication of steel-on-steel contact

Soot generated from the combustion process in diesel engines affect engine tribology. In this paper, two diesel soot samples; from engine exhaust and oil filter are suspended in hexadecane oil and the suspension is used to lubricate a steel ball on steel flat sliding contact at a contact pressure of 1.3 GPa. The friction and wear of the steel flat are recorded. The data are compared with those recorded when the soot is generated by burning ethylene gas. The rationale for the comparatively poor tribology of diesel soot is explored by quantifying the size and shape of primary particles and agglomerates, hardness of single primary soot particles, the crystallinity and surface and near surface chemistry of soot and interparticle adhesion.

Engineering Gold Nanoparticle Interaction by PAMAM Dendrimer

Bare faceted gold nanoparticles (AuNPs) have a tendency to aggregate through a preferred attachment of the 111] surfaces. We have used fully atomistic classical molecular dynamics simulations to obtain a quantitative estimate of this surface interaction using umbrella sampling (US) at various temperatures. To tune this surface interaction, we use polyamidoamine (PAMAM) dendrimer to coat the gold surface under various conditions. We observe a spontaneous adsorption of the protonated as well as nonprotonated PAMAM dendrimer on the AuNP surface. The adsorbed dendrimer on the nanoparticle surface strongly alters the interaction between the nanoparticles. We calculate the interaction between dendrimercoated AuNPs using US and show how the interaction between two bare faceted AuNPs can be tuned as a function of dendrimer concentration and charge (pH dependent) With appropriate choice of the dendrimer concentration and charge, two strongly interacting AuNPs can be made effectively noninteracting. Our simulation results demonstrate a strategy to tune the nanoparticle interaction, which can help in engineering self-assembly of such nanoparticles.

Scaffolds for bone tissue engineering: role of surface patterning on osteoblast response

The fabrication of tissue engineering scaffolds necessitates amalgamation of a multitude of attributes including a desirable porosity to encourage vascular invasion, desired surface chemistry for controlled deposition of calcium phosphate-based mineral as well as ability to support attachment, proliferation, and differentiation of lineage specific progenitor cells. Scaffold fabrication often includes additional surface treatments to bring about desired changes in the surface chemistry. In this perspective, this review documents the important natural and synthetic scaffolds fabricated for bone tissue engineering applications in tandem with the surface treatment techniques to maneuver the biocompatibility of engineered scaffolds. This review begins with a discussion on the fundamental concepts related to biocompatibility as well as the characteristics of the biological micro-environment. The primary focus is to discuss the effects of surface micro/nano patterning on the modulation of bone cell response. Apart from reviewing a host of experimental studies reporting the functionality of osteoblast-like bone cells and stem cells on surface modified or textured bioceramic/biopolymer scaffolds, theoretical insights to predict cell behavior on a scaffold with different topographical features are also briefly analyzed.

Universal flame propagation behavior in packed bed of biomass

This article aims at seeking the universal behavior of propagation rate variation with air superficial velocity (V-s) in a packed bed of a range of biomass particles in reverse downdraft mode while also resolving the differing and conflicting explanations in the literature. Toward this, measurements are made of exit gas composition, gas phase and condensed phase surface temperature (T-g and T-s), and reaction zone thickness for a number of biomass with a range of properties. Based on these data, two regimes are identified: gasificationvolatile oxidation accompanied by char reduction reactions up to 16 +/- 1cm/s of V-s and above this, and char oxidationsimultaneous char oxidation and gas phase combustion. In the gasification regime, the measured T-s is less than T-g; a surface heat balance incorporating a diffusion controlled model for flaming combustion gives and matches with the experimental results to within 5%. In the char oxidation regime, T-g and T-s are nearly equal and match with the equilibrium temperature at that equivalence ratio. Drawing from a recent study of the authors, the ash layer over the oxidizing char particle is shown to play a critical role in regulating the radiation heat transfer to fresh biomass in this regime and is shown to be crucial in explaining the observed propagation behavior. A simple model based on radiation-convection balance that tracks the temperature-time evolution of a fresh biomass particle is shown to support the universal behavior of the experimental data on reaction front propagation rate from earlier literature and the present work for biomass with ash content up to 10% and moisture fraction up to 10%. Upstream radiant heat transfer from the ash-laden hot char modulated by the air flow is shown to be the dominant feature of this model.

Crystal engineering: from molecule to crystal

How do molecules aggregate in solution, and how do these aggregates consolidate themselves in crystals? What is the relationship between the structure of a molecule and the structure of the crystal it forms? Why do some molecules adopt more than one crystal structure? Why do some crystal structures contain solvent? How does one design a crystal structure with a specified topology of molecules, or a specified coordination of molecules and/or ions, or with a specified property? What are the relationships between crystal structures and properties for molecular crystals? These are some of the questions that are being addressed today by the crystal engineering community, a group that draws from the larger communities of organic, inorganic, and physical chemists, crystallographers, and solid state scientists. This Perspective provides a brief historical introduction to crystal engineering itself and an assessment of the importance and utility of the supramolecular synthon, which is one of the most important concepts in the practical use and implementation of crystal design. It also provides a look to the future from the viewpoint of the author, and indicates some directions in which this field might be moving.

Photoluminescence decay rate engineering of CdSe quantum dots in ensemble arrays embedded with gold nano-antennae

We discuss experimental results on the ability to significantly tune the photoluminescence decay rates of CdSe quantum dots embedded in an ordered template, using lightly doped small gold nanoparticles (nano-antennae), of relatively low optical efficiency. We observe both enhancement and quenching of photoluminescence intensity of the quantum dots varying monotonically with increasing volume fraction of added gold nanoparticles, with respect to undoped quantum dot arrays. However, the corresponding variation in lifetime of photoluminescence spectra decay shows a hitherto unobserved, non-monotonic variation with gold nanoparticle doping. We also demonstrate that Purcell effect is quite effective for the larger (5 nm) gold nano-antenna leading to more than four times enhanced radiative rate at spectral resonance, for largest doping and about 1.75 times enhancement for off-resonance. Significantly for spectral off-resonance samples, we could simultaneously engineer reduction of non-radiative decay rate along with increase of radiative decay rate. Non-radiative decay dominates the system for the smaller (2 nm) gold nano-antenna setting the limit on how small these plasmonic nano-antennae could be to be effective in engineering significant enhancement in radiative decay rate and, hence, the overall quantum efficiency of quantum dot based hybrid photonic assemblies.

Scalar transport in diffusion flame wrapped up by an air and fuel side vortex

The present work involves a computational study of soot (chosen as a scalar which is a primary pollutant source) formation and transport in a laminar acetylene diffusion flame perturbed by a convecting line vortex. The topology of soot contours resulting from flame vortex interactions has been investigated. More soot was produced when vortex was introduced from the air side in comparison to the fuel side. Also, the soot topography was spatially more diffuse in the case of air side vortex. The computational model was found to be in good agreement with the experimental work previously reported in the literature. The computational simulation enabled a study of various parameters like temperature, equivalence ratio and temperature gradient affecting the soot production and transport. Temperatures were found to be higher in the case of air side vortex in contrast to the fuel side one. In case of fuel side vortex, abundance of fuel in the vortex core resulted in fuel-rich combustion zone in the core and a more discrete soot topography. Besides, the overall soot production was observed to be low in the fuel side vortex. However, for the air side vortex, air abundance in the core resulted in higher temperatures and greater soot production. Probability density functions (PDFs) have been introduced to investigate the spatiotemporal variation of soot yield and transport and their dependence on temperature and acetylene concentration from statistical view point. In addition, the effect of flame curvature on soot production is also studied. The regions convex to fuel stream side witnessed thicker soot layer. All numerical simulations have been carried out on Fluent 6.3.26. (C) 2013 Elsevier Ltd. All rights reserved.

Nonlinear thermoacoustics of ducted premixed flames: The influence of perturbation convection speed

When a premixed flame is placed within a duct, acoustic waves induce velocity perturbations at the flame's base. These travel down the flame, distorting its surface and modulating its heat release. This can induce self-sustained thermoacoustic oscillations. Although the phase speed of these perturbations is often assumed to equal the mean flow speed, experiments conducted in other studies and Direct Numerical Simulation (DNS) conducted in this study show that it varies with the acoustic frequency. In this paper, we examine how these variations affect the nonlinear thermoacoustic behaviour. We model the heat release with a nonlinear kinematic G-equation, in which the velocity perturbation is modelled on DNS results. The acoustics are governed by linearised momentum and energy equations. We calculate the flame describing function (FDF) using harmonic forcing at several frequencies and amplitudes. Then we calculate thermoacoustic limit cycles and explain their existence and stability by examining the amplitude-dependence of the gain and phase of the FDF. We find that, when the phase speed equals the mean flow speed, the system has only one stable state. When the phase speed does not equal the mean flow speed, however, the system supports multiple limit cycles because the phase of the FDF changes significantly with oscillation amplitude. This shows that the phase speed of velocity perturbations has a strong influence on the nonlinear thermoacoustic behaviour of ducted premixed flames. (C) 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Highly photoactive heterostructures of PbO quantum dots on TiO2

We present a non-hydrolytic sol-gel combustion method for synthesizing nanocomposites of PbO quantum dots on anatase TiO2 with a high surface area. XRD, electron microscopy, DRS, cathodoluminescence and BET were employed for structural, microstructural and optical characterization of the composites. The photocatalytic activity of TiO2 and PbO/TiO2 was investigated and compared with Degussa P-25. The results indicate that the photocatalytic activity of quantum dot dispersed TiO2 is higher than that of bare TiO2 and much higher than that of commercial Degussa P-25. The origin of enhanced photoreactivity of the synthesized material can be assigned to a synergetic effect of high surface area, higher number of active sites and an engineered band structure in the heterostructure. The mechanisms for photocatalytic activity are discussed based on production of photogenerated reactive species. The knowledge gained through this report open up ideal synthesis routes for designing advanced functional heterostructures with engineered band structure and has important implications in solar energy based applications.

Particle size, morphology and color tunable ZnO:Eu3+ nanophosphors via plant latex mediated green combustion synthesis

Efficient ZnO:Eu3+ (1-11 mol%) nanophosphors were prepared for the first time by green synthesis route using Euphorbia tirucalli plant latex. The final products were well characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-visible spectroscopy (UV-Vis), Fourier transform infrared spectroscopy (FTIR), etc. The average particle size of ZnO:Eu3+ (7 mol%) was found to be in the range 27-47 nm. With increase of plant latex, the particle size was reduced and porous structure was converted to spherical shaped particles. Photoluminescence (PL) spectra indicated that the peaks situated at similar to 590, 615, 648 and 702 nm were attributed to the D-5(0) -> F-7(j(j=1,2,3,4)) transitions of Eu3+ ions. The highest PL intensity was recorded for 7 mol% with Eu3+ ions and 26 ml plant latex concentration. The PL intensity increases with increase of plant latex concentration up to 30 ml and there after it decreases. The phosphor prepared by this method show spherical shaped particles, excellent chromaticity co-ordinates in the white light region which was highly useful for WLED's. Further, present method was reliable, environmentally friendly and alternative to economical routes. (c) 2013 Elsevier B.V. All rights reserved.

Thermal Efficiency of LPG and PNG-fired burners: Experimental and numerical studies

In the present work, the thermal efficiency of a conventional domestic burner is studied both experimentally and numerically for liquefied petroleum gas (LPG) and piped natural gas (PNG) fuels. Three-dimensional computational fluid dynamic (CFD) modeling of the steady-state flow, combustion and heat transfer to the vessel is reported for the first time in such burners. Based on the insights from the CFD model concerning the flow and heat transfer, design modifications in the form of a circular insert and a radiant sheet are proposed which are observed to increase thermal efficiency for LPG. For PNG, predictions showed that loading height was a much more important factor affecting efficiency than these design modifications and an optimal loading height could be identified. Experiments confirm these trends by showing an improvement in burner thermal efficiency of 2.5% for LPG with the modified design, and 10% for PNG with the optimal loading height, demonstrating that the CFD modeling approach developed in the present work is a useful tool to study domestic burners. (C) 2013 Elsevier Ltd. All rights reserved.