Some mechanical properties of rigid polyurethane structural foam

The mechanical response of integral-skin rigid polyurethane foam, with an average density of 300 to 700 kg/m³, to constant rate and creep loading was determined. Sandwich specimens were modeled by layers of a core material and two skins, whose secant moduli had been determined experimentally by separate tests and approximated by linear functions of the density. The effective rigidities of the sandwich in tension and flexure were calculated and compared favorably to experimental measurements. The sandwich structure improved the flexural rigidity of homogeneous foam by a factor of more than 2.20. Tensile creep tests of sandwich specimens at relatively low stress levels (up to about 38 percent of their strength) showed that the creep was nonlinear, but a single creep curve could represent creep of specimens of various densities, provided the relative load on them was the same. A limited number of flexural creep tests led to similar conclusions, but the creep rate was smaller than in tension. Results from torsion tests of core material, compressive tests of sandwich specimens, and tension and compression tests of nonskin rigid foam are included in this article.     

Lower energy expenditures in infants from obese biological mothers

Background  

Previous studies in adults have found that a lower resting metabolic rate is a predictor of future body weight gain.     

Exfoliated CrPS4 with Promising Photoconductivity

Layered semiconductors have attracted significant attention due to their diverse physical properties controlled by composition and the number of stacked layers. Herein, large crystals of the ternary layered semiconductor chromium thiophosphate (CrPS₄) are prepared by a vapor transport synthesis. Optical properties are determined using photoconduction, absorption, photoreflectance, and photoacoustic spectroscopy exposing the semiconducting properties of the material. A simple, one‐step protocol for mechanical exfoliation onto a transmission electron microscope grid is developed, and multiple layers are characterized by advanced electron microscopy methods, including atomic resolution elemental mapping confirming the structure by directly showing the positions of the columns of different elements' atoms. CrPS₄ is also liquid exfoliated, and in combination with colloidal graphene, an ink‐jet‐printed photodetector is created. This all‐printed graphene/CrPS₄/graphene heterostructure detector demonstrates a specific detectivity of 8.3 × 10⁸ (D*). This study shows a potential application of both bulk crystal and individual flakes of CrPS₄ as active components in light detection, when introduced as ink‐printable moieties with a large benefit for manufacturing.     

The Influence of Alloy Composition on the Electronic Properties of IV–VI Core/Shell Colloidal Heterostructures

Considering the significant potential of IV–VI colloidal nanostructures in various technologies, this review describes the development and characterization of unique IV–VI derivatives, consisting of core/shell heterostructures, when either the core or the shell (or both) has an alloy composition with the chemical formula PbSe_xS_1−x/PbSe_yS_1−y (0≤x(y)≤1). We describe the synthetic procedures and a thorough investigation of the optical properties of the discussed materials. The experimental evidence is supported by suitable electronic band structure calculations. The special properties are expressed as fine tunability of the electronic properties, chemical and temperature stability, variation of electron phonon interactions, emission Stokes shifts and lifetimes, upon the variation of the composition and/or architecture of the heterostructures. The properties obtained are of significant importance in opto-electronic applications.     

Thermal Isomerization of Azulene. Single-Pulse Shock Tube Investigation

The thermal isomerization of azulene was studied behind reflected shocks in a pressurized driver single-pulse shock tube. The temperature range covered was 1050–1400 K at overall densities of ∼2.5 × 10⁻⁵ mol/cm³. The main reaction of azulene under these conditions is a unimolecular isomerization to naphthalene, but it also isomerizes, although at a much lower rate, to another isomer. The suggested “tetracyclic triene” intermediate structure for the azulene-naphthalene isomerization can lead also to transition states that can describe isomerizations to 1-methylene-1H-indene and 1, 2,3-metheno-1H-indene,2,3-dihydro. Small quantities of C₂H₂, C₄H₂, C₆H₆, and C₆H₅-C≡CH were also found in the post-shock samples, particularly at high temperatures. The Arrhenius parameters of the two high pressure limit rate constants for the isomerization processes are: azulene → naphthalene, k₁ = 10^12.93 exp(–62.8 × 10³/RT) s⁻¹ azulene → second isomer, k₂ = 10^12.42 exp(–69.5 × 10³/RT) s⁻¹ A discussion of the mechanism for these isomerization processes is presented.     

A theoretical model for effective thermal conductivity (ETC) of particulate beds under compression

Thermal conductivity of particulate beds is an important property for many industrial handling processes as well as for storage of particulate materials. This paper presents a new theoretical model that is based on heat transfer between particles in three modes: heat conduction through contact area, heat conduction through voids and radiation through voids. The model is further adjusted in order to obtain effective thermal conductivity of a particulate bed by using empirical augmentation factors for the heat transfer coefficient of each one of the heat transfer mechanisms. Comparison of the results predicted by the semi-empirical model to our experimental results show good agreement. The theoretical model was investigated to examine the effect of various parameters (such as: particle elasticity and surface roughness, particle and gas thermal conductivity and particle diameter), on the effective thermal conductivity of various particulate beds. Our results show the significant effect of the contact area (that is a clear function of the compression load) between particles on the effective thermal conductivity.     

Well‐Aligned ZnO Nanowire Arrays Fabricated on Silicon Substrates

Arrays of well‐aligned single‐crystal zinc oxide (ZnO) nanowires of uniform diameter and length have been synthesized on a (100) silicon substrate via a simple horizontal double‐tube system using chemical vapor transport and condensation method. X‐ray diffraction and transmission electron microscopy (TEM) characterizations showed that the as‐grown nanowires had the single‐crystal hexagonal wurtzite structure with detectable defects and a <0002> growth direction. Raman spectra revealed phonon confinement effect when compared with those of ZnO bulk powder, nanoribbons, and nanoparticles. Photoluminescence exhibited strong ultraviolet emission at 3.29 eV under 355 nm excitation and green emission at 2.21 eV under 514.5 nm excitation. No catalyst particles were found at the tip of the nanowires, suggesting that the growth mechanism followed a self‐catalyzed and saturated vapor–liquid–solid (VLS) model. Self‐alignment of nanowires was attributed to the local balance and steady state of vapor flow at the substrate. The growth technique would be of particular interest for direct integration in the current silicon‐technology‐based optoelectronic devices.     

Close contacts with the endoplasmic reticulum as determinants of mitochondrial Ca2+ responses

The spatial relation between mitochondria and endoplasmic reticulum (ER) in living HeLa cells was analyzed at high resolution in three dimensions with two differently colored, specifically targeted green fluorescent proteins. Numerous close contacts were observed between these organelles, and mitochondria in situ formed a largely interconnected, dynamic network. A Ca2+-sensitive photoprotein targeted to the outer face of the inner mitochondrial membrane showed that, upon opening of the inositol 1,4,5-triphosphate (IP3)-gated channels of the ER, the mitochondrial surface was exposed to a higher concentration of Ca2+ than was the bulk cytosol. These results emphasize the importance of cell architecture and the distribution of organelles in regulation of Ca2+ signaling.     

Beam heating of a moderately thick cold stage specimen in the SEM/STEM

A mathematical model for the electron beam heating problem of thick specimens (thickness less than the electron range) in the cold stage of SEM/STEM is proposed, and an analytic solution for the problem is provided. The use of the model is demonstrated by calculating the maximum temperature rise in a red blood cell mounted on a metallized polymer film, for a large range of operating conditions. With a probe size of 20 nm and probe current of less than 10 nA the temperature rise was found to be on the order of 5 K. This temperature rise increases proportionally to the probe current if all other variables are kept constant. Although the amount of energy dissipated in thick specimens is large relative to thin films, temperature rise can be minimized by mounting the thick sample on a metal stub or on a metallized film thus providing large heat transfer area through a high thermal conductivity substrate. Of course our calculations only have meaning if the specimen is in good thermal contact with the underlying support.     

Longitudinal tensile failure of unidirectional fibrous composites

This paper presents a theoretical treatment of the tensile strength of a unidirectional fibrous composite, subjected to a tensile load in the fibre direction. The fibres are treated as having a statistical strength distribution which results in fibre failure prior to composite failure. The failure geometry of the model is similar to the observed geometry of fractured glass/epoxy and glass/polyester composites. Failure criterion is established and the strength is shown to decrease as the length of the specimen is increased. This size effect is very small.