Search results for " foam"

Mechanical properties of rigid polyurethane foams at room and cryogenic temperatures

2011

Studies on the effect of the foams’ polymeric matrix’ properties on the tension and compression properties of pour rigid polyurethane (PUR) foams, apparent core density 65—70 kg/m3, at 296 and 77 K were carried out. PUR foams were produced by the hand mixing method from polyol systems that comprised polyether, polyester polyols, and chain extenders. To produce PUR foams, crude MDI was used, and Solkane 365 mfc/227 ea was used as a blowing agent. The molecular weight per branching unit (Mc) of the polymeric matrix of PUR foams was varied in the range 300—1150. Cohesion energy densities of the blocks forming the polymeric matrix were calculated. The effect of Mc on the formation of hydrogen …

Estimation of the effective diffusivity of blowing agents in closed-cell low-density polyurethane foams based on thermal aging data

2021

Abstract Low-density closed-cell polyurethane (PU) foams are applied as thermal insulation materials due to their low thermal conductivity imparted by that of the physical blowing agent (PBA) used in foam production. However, foam conductivity tends to gradually increase with time, primarily due to changes in the gas composition in foam cells brought about by gas diffusion. To enable predicting the variation of conductivity during the service life of foam insulation, gas diffusivities are usually determined by measuring the gas composition in foams at different aging times. This study considers an alternative approach of estimating the effective diffusivities of gases in PU foams, which is …

Electroburning of few-layer graphene flakes, epitaxial graphene, and turbostratic graphene discs in air and under vacuum

2015

Graphene-based electrodes are very promising for molecular electronics and spintronics. Here we report a systematic characterization of the electroburning (EB) process, leading to the formation of nanometer-spaced gaps, on different types of few-layer graphene (namely mechanically exfoliated graphene on SiO2, graphene epitaxially grown on the C-face of SiC and turbostratic graphene discs deposited on SiO2) under air and vacuum conditions. The EB process is found to depend on both the graphene type and on the ambient conditions. For the mechanically exfoliated graphene, performing EB under vacuum leads to a higher yield of nanometer-gap formation than working in air. Conversely, for graphene…

Micro‐tomographic characterization of composite recycled glass‐silicone foams for applications in civil engineering

2019

Noninvasive X‐ray micro‐computed tomography was applied for a complete quantitative and qualitative analysis of the cellular structure of composite foams constituted by a silicone matrix and a glass production waste filler. Composite foams with different glass filler weight content in the range 0–80% were synthesized and characterized. The tomographic analysis was employed in order to assess the structural heterogeneities, void fraction values, and bubble size distribution for all composite foams. The 3D micro‐CT images analysis, performed at different cross‐sections, highlighted heterogeneous cell growth or more elongated cells in the case of low and high filler content foams, respectively…

Composites of Graphene with Large Aromatic Molecules

2009

GrailQuest and HERMES: hunting for gravitational wave electromagnetic counterparts and probing space-time quantum foam

2021

GrailQuest (Gamma-ray Astronomy International Laboratory for Quantum Exploration of Space-Time) is an ambitious astrophysical mission concept that uses a fleet of small satellites whose main objective is to search for a dispersion law for light propagation in vacuo. Within Quantum Gravity theories, different models for space-time quantization predict relative discrepancies of the speed of photons w.r.t. the speed of light that depend on the ratio of the photon energy to the Planck energy. This ratio is as small as 10-23 for photons in the γ- ray band (100 keV). Therefore, to detect this effect, light must propagate over enormous distances and the experiment must have extraordinary sensitivi…

Is There a C-Function in 4D Quantum Einstein Gravity?

2016

We describe a functional renormalization group-based method to search for ‘C-like’ functions with properties similar to that in 2D conformal field theory. It exploits the mode counting properties of the effective average action and is particularly suited for theories including quantized gravity. The viability of the approach is demonstrated explicitly in a truncation of 4 dimensional Quantum Einstein Gravity, i.e. asymptotically safe metric gravity.

The Quantum Scalar Field in Spherically Symmetric Loop Quantum Gravity

2013

We consider the quantization of a spherically symmetric gravitational system coupled to a massless scalar field within the loop quantum gravity framework. Our results rely on the uniform discretizations method developed during the last years. We minimize the associated discrete “master constraint” using a trial state whose gravitational part is peaked around the classical Schwarzschild solution.

The quantum, the geon, and the crystal

2015

Effective geometries arising from a hypothetical discrete structure of space-time can play an important role in the understanding of the gravitational physics beyond General Relativity. To discuss this question, we make use of lessons from crystalline systems within solid state physics, where the presence of defects in the discrete microstructure of the crystal determine the kind of effective geometry needed to properly describe the system in the macroscopic continuum limit. In this work we study metric-affine theories with non-metricity and torsion, which are the gravitational analog of crystalline structures with point defects and dislocations. We consider a crystal-motivated gravitationa…

Black hole state counting in loop quantum gravity: a number-theoretical approach

2008

4 pages, 1 figure.-- PACS nrs.: 04.70.Dy, 04.60.Pp.-- ArXiv pre-print available at: http://arxiv.org/abs/0802.4077