Helium pycnometry

Helium pycnometry probes the volume of a sample that is inaccessible to helium atoms hereby a volumetric method is usually applied that is also implemented in different 

Using the ideal gas equation, the volumes of the two chambers involved, and the equilibrium gas pressures prior and after the valve has been opened, the specific volume Usk of the sample that is not accessible to helium can be calculated  

For untreated aerogels that possess by definition a well-accessible pore network, the volume determined corresponds to the specific volume of the aerogel backbone, i.e., the inverse of the aerogel backbone density this is an important figure for the calculation of the 

In case of carbon aerogels, where additional microporosity is created uptm the carbonization of the organic precursor, the skeletal volume is representing the volume of the amorphous carbon phase when the porosity is well accessible for the helium. 

Comment. In many cases the density of the solid phase can well be approximated by using the respective literature values for the given chemical composition. For nonmacroporous samples with well-accessible porosity, the total pore volume and thus the backbone density can also be determined from the macroscopic density of the (degassed ) sample and the respective nitrogen sorption isotherm (see Chap. 21.6). 

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Plrue true density of the particle (usually determined by helium pycnometry)... 

A plot of density versus pore radius, from the data in Table 21.2, is shown in Fig. 21.3. The horizontal line indicates the true density obtained by helium pycnometry. This higher density by gas displacement reflects the volume of pores smaller than about 18 A. 

Helium pycnometry can also be used to obtain an estimate of porosity providing that the porous sample is sealed to prevent gas penetration. There are experimental problems that need to be addressed to ensure that the seal is effective and that it doesn t significantly alter the volume or weight of the sample which are beyond the scope of this study. 

Sample Sample code Apparent density (Buoyancy method) (gem 3) Apparent porosity (%) Theoretical porosity (%) True density (Helium pycnometry) (gem 3)... 

Helium is often used in adsorption manometry for the determination of the dead space volume (see Chapter 3), but this procedure is based on the presupposition that the gas is not adsorbed at ambient temperature and that it does not penetrate into regions of the adsorbent structure which are inaccessible to the adsorptive molecules. In fact, with some microporous adsorbents, significant amounts of helium adsorption can be detected at temperatures well above the normal boiling point (4.2 K). For this reason, the apparent density (or so-called true density ) determined by helium pycnometry (Rouquerol et al., 1994) is sometimes dependent on the operational temperature and pressure (Fulconis, 1996). 

It can be calculated if we have at least an approximate value for V, e.g. by determining the sample volume by means of Helium pycnometry and assuming that Helium is not adsorbed ... 

Determining again the volume of the solid sample V by means of Helium pycnometry a nd using eq. (4a) together with... 

Cylindrical pellets of four industrial and laboratory prepared catalysts with mono- and bidisperse pore structure were tested. Selected pellets have different pore-size distribution with most frequent pore radii (rmax) in the range 8 - 2500 nm. Their textural properties were determined by mercury porosimetry and helium pycnometry (AutoPore III, AccuPyc 1330, Micromeritics, USA). Description, textural properties of catalysts pellets, diameters of (equivalent) spheres, 2R, (with the same volume to geometric surface ratio) and column void fractions, a, (calculated from the column volume and volume of packed pellets) are summarized in Table 1. Cylindrical brass pellets with the same height and diameter as porous catalysts were used as nonporous packing. 

A comparison of true particle density, apparent particle density, and bulk density can provide information on total porosity, interparticle porosity, and intraparticle porosity. Methods include true particle density measurements via helium pycnometry, mercury intrusion porosimetry, and poured and tapped bulk density. 

Vanadia-doped sihca aerogels with densities between 200 kg m and 400 kg m were investigated via helium pycnometry, nitrogen adsorption and small angle X-ray scattering. While evaluating the scattered intensity with the two-phase-media model, the... 

The true density of the talc samples was measured using helium pycnometry, and the data is presented in Table IV. The true densities were similar for the three lots and the minor differences can be attributed to die impurities in the lots. 

The true density of a solid is the average mass per unit volume, exclusive of all voids that are not a fundamental part of the molecular packing arrangement. This density parameter is normally measured by helium pycnometry, where the volume occupied by a known mass of powder is determined by measuring the volume of gas displaced by the powder. The true density of a solid is an intrinsic property characteristic of the analyte and is determined by the composition of the unit cell. 

The true density (measured by helium pycnometry) of titanium dioxide differs with the polymorphic state of the material. Rutile is the most dense (4.2S g/mL), followed by brookite (4.12 g/mL) and anatase (3.89 g/mL). 

The porosity and pore stmcture can be also determined with mercury porosim-etry, as well as with helium flow method proposed by Feldman [48]. However, a more adequate method of the total porosity measmement is helium pycnometry [57]. The applicability of the mercury intmsion porosimetry covers the pore diameters from 3 nm to about 10 nm, while the capillary condensation— from 4 to 50 nm respectively. 

Silica aerogels are amorphous materials. They have a skeletal density, as measured by Helium pycnometry [102] 2 g cm, close to that of amorphous silica (2.2 g cm ). They typically have a pore volume above 90% of their whole monolith volume. Some ultraporous... 

That paper also showed that the kinetics were accelerated significantly by using the tin-based organometallic catalyst (gelation times stand for some minutes to some hours instead of some days) and, above all, permitted to know more about the internal structure of this type of aerogels. They appeared to consist of spherical dense particles with densities, measured by helium pycnometry, close to 1.45 g/cm forming (a) a fibrillar solid network,... 

Light scattering Light scattering Inelastic neutron scattering Adsorption Helium pycnometry... 

Figure 21.22. Schematic setup for helium pycnometry with an initial pressure in the sample chamber of zero (only the difference in gas pressure between the two chambers is relevant therefore any actual present offset can be subtracted from the pressures in the system). After a difference in helium pressure (prc) is established between the reference and the sample chamber (volume Vrc and Vsc, respectively), the valve between the chambers is opened. At equilibrium the pressure in the two chambers is equal. The setup is kept under thermostated conditions.

When actually applying helium pycnometry, care has to be taken that the sample is well degassed and the setup used is at constant temperature. If not, artifacts resulting in backbone densities that are up to 50% off can be the consequence. 

Helium pycnometry Characterization method permitting to determine the skeletal density and specific volume of a porous sample by measuring the pressure change of helium in a calibrated volume HEMA Hydroxyethyl methacrylate... 

Bulk and apparent density Helium pycnometry Mercury porosimetry Liquid displacement Surface energy Thermal analysis tests Tempera ture-progra m med desorption and reaction Calorimetry... 

Although more labor-intensive and less efficient, information on the densification kinetics and densification can also be obtained from density measurements on different individual samples as a function of time for otherwise identical sintering conditions. Bulk density measurements on <92% dense sintered samples containing open porosity can be determined from the measured mass and dimensions of the compact, while Archimedes method works well for closed pore, >92% dense bodies. The density of closed pore samples can also be determined by pycnome-try (e.g., helium pycnometry),, 37 mercury porosimetry, and by the sink-float method (i.e., whereby the buoyancy of the sample is assessed and compared in different density liquids). Density can also be estimated from micrographs using quantitative stereology. 

The skeletal density was measured with helium pycnometry (Micromeritic Accupyc 1330) with 10 purges and the run precision software (with at least a 0.05% volume tolerance level). 

On dry gels, standard characterization techniques for porous media are used, several of which have been described in Volume 2 of this series helium pycnometry for pore volume determination (Section 6.3.1.2) as well as nitrogen adsorption at 77 K for surface area (Section 6.3.2.2, BET method), for microporosity (Section 6.3.3.2, Dubinin-Radushkevich method), for pore size distribution (Section 6.3.3.3, BJFl method), and for total pore volume (Section 6.3.3.4). When characterizing gels by nitrogen adsorption, other methods are also used for data interpretation, for example, the t-plot method for microporosity (Lippens and de Boer, 1965) and the Dollimore-Heal method (Dollimore and Heal, 1964) or Broekhoff-de Boer theory for mesoporosity (Lecloux, 1981). 

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