A resonance based laser ultrasonics technique is explored for the characterization of low density nanoporous gold foams. Laser generated zero group velocity (ZGV) lamb waves are measured in the foams using a Michelson interferometer. The amplitude spectra obtained from the processed time-domain data are analyzed using a theoretical model from which the foam Young’s modulus and Poisson’s ratio are obtained. The technique is non-contact and nondestructive, and the ZGV resonance modes are spatially localized, allowing for spatial mapping of the bulk sample properties. The technique may be suitable for process control monitoring and mechanical characterization of low density nanoporous structures. 相似文献
ABSTRACT Lignin, extracted from sugarcane bagasse, was used as a partial phenol substitute in phenolic closed cell foams. The thermal stability of phenolic and lignophenolic foams were studied using thermogravimetric (TG) and differential scanning calorimetry (DSC) techniques, under air and nitrogen atmospheres. The results of apparent densities (Dapp) are also reported for both foams. The thermal analyses data showed that the decomposition depends on the atmosphere, that is, this process is not only a thermal one and, that it is feasible to replace part of the phenol by lignin in closed cell foams. Regarding the apparent density, that replacement was extremely advantageous because the Dapp value obtained placed the lignophenolic foam in the structural foam class. 相似文献
In aqueous systems, partially hydrophobic particles are known to stabilize foams even in the absence of any added surfactant. This paper shows that the same principle can be applied to polymeric systems: particles that are partially wetted by a polymer melt can stabilize a foam of that polymer. The foam stability is attributable to the adsorption of the particles at the air/polymer interface. Remarkably, stable foams are realized even from polymers that are liquid at room temperature, and hence are otherwise unfoamable. The implications of this result to practical foaming operations are discussed.
A single film (typical of a film in a foam) moving in a confined geometry (i.e. confined between closely spaced top and bottom plates) is analysed via the viscous froth model. In the first instance the
film is considered to be straight (as viewed from above the top plate) but is not flat. Instead it is curved (with a circular
arc cross-section) in the direction across the confining plates. This curvature leads to a maximal possible steady propagation
velocity for the film, which is characterised by the curved film meeting the top and bottom plates tangentially. Next the
film is considered to propagate in a channel (i.e. between top and bottom plates and sidewalls, with the sidewall separation exceeding that of the top and bottom plates). The
film is now curved along as well as across the top and bottom plates. Curvature along the plates arises from viscous drag
forces on the channel sidewall boundaries. The maximum steady propagation velocity is unchanged, but can now also be associated
with films meeting channel sidewalls tangentially, a situation which should be readily observable if the film is viewed from
above the top plate. Observed from above, however, the film need not appear as an arc of a circle. Instead the film may be
relatively straight along much of its length, with curvature pushed into boundary layers at the sidewalls. 相似文献