Porous materials measurements

Another type of experiment to measure separately from other factors consists of saturating packings made from porous materials using a volatile... 

Pressure Measurement Although successful pressure-measurement probes or taps have been fabricated by using porous materials, the most universally accepted pressure tap consists of a purged tube projecting into the bed as nearly vertic ly as possible. Minimum internal diameters are 1 to 2 cm (V2 to 1 in). A purge rate of at least 0.9 m/s (3 ft/s) is usually required. Pressure measurements taken at various heights in the bed are used to determine bed level. 

In any leaf test program there is always a question as to what vacuum level should be used. With very porous materials, a vacuum in the range of 0.1 to 0.3 bar (3 to 9 in Hg) should be used, and, except for thermal-drying apphcations using hot air, the vacuum level should be adjusted to give an air rate in the range of 450 to 900 mVm h (30 to 40 cfm/ft measured at the vacuum. 

The most important evaluation of an ANG storage systems performance is the measurement of the amount of usable gas which can be delivered from the system. This is frequently defined as the volume of gas obtained from the storage vessel when the pressure is reduced from the storage pressure of 3.5 MPa (35 bar) to one bar, usually at 298 K. This parameter is referred to as the delivered V/V and is easy to determine directly and free from ambiguity. Moreover, it is independent of the ratio of gas adsorbed to that which remains in the gaseous state. To determine the delivered V/V an adsorbent filled vessel of at least several hundred cubic centimeters is pressurized at 3.5 MPa and allowed to cool under that pressure to 298 K. The gas is then released over a time period sufficient to allow the bed temperature to return to 298 K. A blank, where the vessel is filled with a volume of non-porous material, such as copper shot. 

ITie BET method is the most widely used procedure for determining the surface area of porous materials. In this chapter, BET results were obtained from single point measurements using a Micromeritics Flowsorb II 2300 surface area analyzer. A mixture of nitrogen in helium (30 70 mole percentage) was used. Although this simple method is not quantitative for the microporous materials studied in section 5, it still allows qualitative comparisons to be made. 

Filter life Measure of the duration of a filter s useful service. This is based on the amount of standard contaminant required to cause differential pressure to increase to an unacceptable level-typically 2-4 times the initial differential pressure, a 50-80% drop in initial flow, or a downstream measure of unacceptable particulate. Filter media A porous material for separating suspended particulate matter from fluid. 

The substantial literature on the bulk characterization of porous materials using conventional techniques provides a useful foundation as a starting point for overlapping the basic physics of traditional materials analysis with the parameters that can be measured using NMR. 

NMR signals are highly sensitive, via a number of different mechanisms, to the physical and chemical properties of porous materials. Using the set of NMR-based measurement methods that we have developed, it is possible to non-invasively and non-destructively characterize both the microstructural properties of the materials and relaxation properties of fluids imbibed into these materials. 

Extensive experimental techniques have been developed for porous material characterization [1], including direct imaging [2-5] and bulk measurement techniques for the statistical properties of the pore space. NMR is one such bulk measurement that is both non-destructive and compatible with large samples. 

Since it was proposed in the early 1980s [6, 7], spin-relaxation has been extensively used to determine the surface-to-volume ratio of porous materials [8-10]. Pore structure has been probed by the effect on the diffusion coefficient [11, 12] and the diffusion propagator [13,14], Self-diffusion coefficient measurements as a function of diffusion time provide surface-to-volume ratio information for the early times, and tortuosity for the long times. Recent techniques of two-dimensional NMR of relaxation and diffusion [15-21] have proven particularly interesting for several applications. The development of portable NMR sensors (e.g., NMR logging devices [22] and NMR-MOUSE [23]) and novel concepts for ex situ NMR [24, 25] demonstrate the potential to extend the NMR technology to a broad application of field material testing. 

Electrical resistance blocks Measures resistance resulting from a gradient between the sensor and the soil higher resistance indicates lower soil moisture Consists of electrodes embedded in a gypsum, nylon, or fiberglass porous material... 

PALS is based on the injection of positrons into investigated sample and measurement of their lifetimes before annihilation with the electrons in the sample. After entering the sample, positron thermalizes in very short time, approx. 10"12 s, and in process of diffusion it can either directly annihilate with an electron in the sample or form positronium (para-positronium, p-Ps or orto-positronium, o-Ps, with vacuum lifetimes of 125 ps and 142 ns, respectively) if available space permits. In the porous materials, such as zeolites or their gel precursors, ort/zo-positronium can be localized in the pore and have interactions with the electrons on the pore surface leading to annihilation in two gamma rays in pick-off process, with the lifetime which depends on the pore size. In the simple quantum mechanical model of spherical holes, developed by Tao and Eldrup [18,19], these pick-off lifetimes, up to approx. 10 ns, can be connected with the hole size by the relation ... 

As described before, the pore size of porous material ranges widely from atomic size to millimeter order. Different pore sizes are required for different applications of porous materials. Most porous materials do not have uniform pores. Pore size distribution is also an important property. Narrow pore size distribution, i.e., uniform pore size, is required for instance for filters and bioreactor beds. Mercury porosimetry and gas adsorption methods are commonly used to measure pores size and pores distribution. 

Pores are found in many solids and the term porosity is often used quite arbitrarily to describe many different properties of such materials. Occasionally, it is used to indicate the mere presence of pores in a material, sometimes as a measure for the size of the pores, and often as a measure for the amount of pores present in a material. The latter is closest to its physical definition. The porosity of a material is defined as the ratio between the pore volume of a particle and its total volume (pore volume + volume of solid) [1]. A certain porosity is a common feature of most heterogeneous catalysts. The pores are either formed by voids between small aggregated particles (textural porosity) or they are intrinsic structural features of the materials (structural porosity). According to the IUPAC notation, porous materials are classified with respect to their sizes into three groups microporous, mesoporous, and macroporous materials [2], Microporous materials have pores with diameters < 2 nm, mesoporous materials have pore diameters between 2 and 50 nm, and macroporous materials have pore diameters > 50 nm. Nowadays, some authors use the term nanoporosity which, however, has no clear definition but is typically used in combination with nanotechnology and nanochemistry for materials with pore sizes in the nanometer range, i.e., 0.1 to 100 nm. Nanoporous could thus mean everything from microporous to macroporous. 

Pressure Measurement Although successful pressure measurement probes or taps have been fabricated by using porous materials, the most universally accepted pressure tap consists of a purged tube... 

This method is based on the principle of capillary equilibrium, providing a nondestructive technique in which the total porosity and capillary pressure-saturation data are measured [198,199]. The idea is that when two partially saturated porous materials are in contact, the system moves toward an equilibrium state in which the capillary pressure of a liquid in one of fhe samples is the same for fhe ofher material. 

Another technique, capillary flow porometry has been developed by Porous Materials Inc. ° to characterize battery separators.The instrument can measure a number of characteristics of battery separators such as size of the pore at its most constricted part, the largest pore size, pore size distribution, permeability, and envelope surface... 

Figure 18. Schematic drawing depicting SECM measurement of (a) molecular transport within a porous material and (b) electrochemical activity on one electrode In a battery array.

As discussed above, hysteresis loops can appear in sorption isotherms as result of different adsorption and desorption mechanisms arising in single pores. A porous material is usually built up of interconnected pores of irregular size and geometry. Even if the adsorption mechanism is reversible, hysteresis can still occur because of network effects which are now widely accepted as being a percolation problem [21, 81] associated with specific pore connectivities. Percolation theory for the description of connectivity-related phenomena was first introduced by Broad-bent et al. [88]. Following this approach, Seaton [89] has proposed a method for the determination of connectivity parameters from nitrogen sorption measurements. 

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