Radiative heat transfer in enhanced hydrogen outgassing of glass

Abstract

This paper explores the physical mechanisms responsible for experimental observations that led to the definition of "photo-induced hydrogen outgassing of glass". Doped borosilicate glass samples were placed inside an evacuated silica tube and heated in a furnace or by an incandescent lamp. It was observed that hydrogen release from the glass sample was faster and stronger when heated by an incandescent lamp than within furnace. Here, sample and silica tube were modeled as planeparallel slabs exposed to furnace or to lamp thermal radiation. Combined conduction, radiation, and mass transfer were accounted for by solving the one-dimensional transient mass and energy conservation equations along with the steady-state radiative transfer equation. All properties were found in the literature. The experimental observations can be qualitatively explained based on conventional thermally activated gas diffusion and by carefully accounting for the participation of the silica tube to radiation transfer along with the spectral properties of the silica tube and the glass samples. In brief, the radiation emitted by the incandescent lamp is concentrated between 0.5 and 3.0 µm and reaches directly the sample since the silica tube is nearly transparent up to 3.5 µm. On the contrary, for furnace heating at 400 o C, the silica tube absorbs a large fraction of the incident radiation which reduces the heating rate and the H 2 release rate. However, between 0.8 and 3.2 µm undoped borosilicate does not absorb significantly. Coincidentally, Fe 3 O 4 doping increases the absorption coefficient and also reacts with H 2 to form ferrous ions which increase the absorption coefficient of the sample by two orders of magnitude. Thus, doped and reacted 1 samples heat up much faster when exposed to the heating lamp resulting in the observed faster response time and larger H 2 release rate.Keywords: hydrogen storage, photo-induced gas diffusion, hollow glass microspheres, outgassing NOMENCLATURE c p Specific heat at constant pressure (J/kg.K)

A hybrid hydrolytic hydrogen storage system based on catalyst-coated hollow glass microspheres

“…Some researchers use K 0 and Q K while n is set equal to 1 [17,20,27,36], whereas others use the standard Arrhenius law for which n = 0 [18,21,37]. Some researchers use K 0 and Q K while n is set equal to 1 [17,20,27,36], whereas others use the standard Arrhenius law for which n = 0 [18,21,37].…”

“…A thermally driven release requires temperatures of 200-400°C (473.15-673.15 K), depending on required hydrogen flow rate, sphere material and sphere dimension, and is also limited by the thermal conductivity of the spheres [26]. Another option is to utilize an infrared lamp, which has been under research since 1998 mainly but not only by the group of Shelby [20,22,24,25,27,28].…”

“…where the constants K 0 , n and Q K are determined empirically. Some researchers use K 0 and Q K while n is set equal to 1 [17,20,27,36], whereas others use the standard Arrhenius law for which n = 0 [18,21,37]. In general, hydrogen permeability increases with the amount of glass network formers G[mol%] and decreases with the amount of modifying oxides M[mol%].…”

“…For calculations of the extraction behaviour of S38 and optimized glass spheres, the relations of Shelby (1974) is used [39], which was also used by Kitamura (2009) [27]. The time t 1/e when the pressure inside the sphere has dropped to 1/e times the initial pressure p 0 should be in the range of 100 days [23].…”

Fabrication of zinc-loaded hollow glass microspheres (HGMs) for hydrogen storage

“…This can also lead to partial filling of the hydrogen gas during adsorption. In order to improve the thermal conductivity, modifications of the HGMs by adding metals/metal oxides of transition metals have been done . In recent years, many attempts have been made to improve the thermal properties of HGMs by doping transition elements like, Fe, Co and Zn and thereby improving the rate of hydrogen release after hydrogen storage .…”