Pores irreversible changes

For compressed samples with other content of MWNT the temperature dependences p(T) h Et(T) are like to that shown in Fig. 4. Irreversible changes of p h Et in this case can be explained by the structure relaxation processes and by increase of pore and microcracks concentration. As it follows from the data obtained, dependence Et(T) is insensitive to pores and microcracks, while p(T) is sensitive to these. 

Another interesting feature is the reversibility of the water isotherms at low p/p. This is in marked contrast to the low-pressure hysteresis exhibited by water isotherms determined on most other forms of dehydroxylated silicas.The fact that there is no apparent tendency for rehydroxylation suggests that water does not easily penetrate into the intracrystalline pores of Silicalite or HZSM-5 to any great extent. However, in the work of Llewellyn et al. (1996), water was condensed on the Silicalite-I sample at p/p° = 1.0 and this did produce an irreversible change in the low pressure region of the water isotherm. 

Studies on other materials show that MIP determines the width distribution of pore entrances and not of the pores themselves (D34). The intrusion of mercury may also coarsen the pore structure this need only imply that, at the higher pressures employed, some of the foils of the gel are displaced so that some pores are widened and entered while neighbouring ones are closed up. The combined result of these processes would be to produce a distribution narrower than that existing before the intrusion began, and a value for the porosity at maximum pressure that corresponded to a minimum pore width before intrusion of less than 3.5 nm. Experiments in which the mercury was removed and subsequently reintruded have indicated that the structure is usually not altered in the case of Portland cement pastes, though it is in that of pastes of composite cements (F35,D32), but cannot show whether an irreversible change occurred during the first intrusion. 

First of all, the residual water in the pores of B2 G2 is responsible for these effects. The adsorbed water reduces the pore size and their whole volume as well. Additionally, irreversible changes of the microstructure of Bl Gl due to the heat treatment, such as the breakdown of pore walls, the collapse of small pores or the degradation of hydrate phases, have to be associated with the findings [5, 6]. The influence of the contact angle should also be considered, as it is not constant as assumed in the calculations [1,12]. 

Scattering techniques are non intrusive, non-destructive and have particular advantages in the characterisation of the surface and porous properties of materials containing either closed or molecular sized pores and where outgass-ing pre-treatment may result in irreversible changes in microstructure. Measurements may be carried out, indeed, in the presence of a gas or with hydrated materials and there is no need to evacuate the sample. 

Microporous manbranes are sometimes classified as isotropic, with uniform pore size through the body of membrane, or arusotropic, where pores are changing in size of a membrane surface to another. In microporous membranes, occur despite retention of aU particles larger than the pore size of the manbrane, the particles of the same pore size can pass through them and block than, irreversibly sealing the membrane [6]. 

Mercury and water porosimetry measurements have shown that the GDL pore structure changes during lifetime tests. Large pore (30-60 um diameter) volume has decreased, while small-pore volume increases. The loss in large-pore volume is likely due to irreversible fuel cell compression [118]. 

In addition, for materials with very low strength, characterization techniques normally deemed quite gentle can become destructive. One example is the widespread use of gas adsorption to measure pore size which produces irreversible changes in ultrahigh porosity silica aerogels (Scherer et al. 1995) and is likely to run into similar issues with characterization of silicon aerocrystals (Canham et al. 1994). 

The carbon powder corrosion in the MPL can also occur in the environment of an operating PEMFC. Porosimetry measurements indicate that carbon is lost from the MPL dining operation. " Porosimetry measurements have also shown that the GDL pore stmcture changes during lifetime tests. Large pore (30-60 pm diameter) volume has decreased, while small-pore volume increases. The loss in large-pore volume is probably due to irreversible compression due to cell compression. However, there are ordy a few hterature papers that examine mechanical degradation and review the effect of compression of the GDL on PEMFC performance. ... 

A discussion of the adsorption of water on oxides would be incomplete without some reference to the irreversible effects which are often encountered when samples of oxide, hydroxide or oxide-hydroxide are exposed to the vapour. These effects ( low-temperature ageing ), which manifest themselves in changes in surface area, in pore structure and sometimes in the lattice structure itself, are complex and difficult to reproduce exactly. ... 

Deactivation is brought about primarily by the continual and irreversible buildup of deposits within the catalyst pores. Loss of activity can be attributed to both the obstruction of catalyst pores by deposits and the chemical changes that occur when the deposited metal interacts with the original active sites on the catalyst. The former restricts the access of... 

PH adjustment / filtration The pH adjustment/filtration test evaluates the effect of pH change and filtration on the toxicity of substances associated with filterable material, focusing on irreversible chemical reactions. Effluent samples, at pH 3, 11 and / are filtered (using positive pressure) through a glass fibre filter (1.0 p. pore size). The pH of each filtered sample is re-adjusted to pH / prior to testing. 

The action of steam may have a deleterious effect on catalytic materials. For example, transport of silica can lead to loss of support material or to the encapsulation of the active phase. Steaming may also change the pore structure of the support. As somewhat lower temperatures the action of water vapour may result in an irreversible decrease in the surface area. 

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