Contrast-matching small-angle neutron scattering evidence for the absence of a connected pore system in Pittsburgh No. 8 coal

Small-Angle Scattering From Confined and Interfacial Fluids

2016

Understanding the influence of confinement on the structure and thermodynamic properties of supercritical fluids in pores of different surface chemistry and topology is fundamental to the successful development and optimization of the variety of technologies involving fluid-solid interactions. Adsorption behavior of supercritical fluids (SCFs) is fundamentally different from that of subcritical vapors and this chapter begins with a general description of the specifics of the supercritical adsorption. Presented examples illustrate the capability of SAS methods to deliver unique, pore-size-dependent information on the properties of supercritical CO 2 , hydrogen, methane, and other fluids confined in pores of different engineered and natural porous solids. Applications of SAS for studying pore interconnectivity and structural stability of porous materials under pressure are also discussed.

Section: Pore Interconnectivity and Accessibility To Fluids In Coal Amentioning

confidence: 99%

Supercritical Fluids in Confined Geometries

Small-Angle Scattering From Confined and Interfacial Fluids

2016

“…Because of the large difference between the coherent scattering length of deuterium and hydrogen, isotopic mixtures of various solvents can be used to contrast match the SLD of a rock matrix and eliminate the matrix-pore scattering. Contrast matching with mixtures of (D 2 O + H 2 O) has been widely employed for SANS characterization of the structure of various coals, shales, and sandstone [41][42][43][44][45][46]. Isotopic mixtures of toluene (C 7 D 8 + C 7 H 8 ) were used to determine closed porosity in coal chars [47], and isotopic mixtures of methanol (CD 4 O + CH 4 O) were used to determine the neutron SLD and structure of kerogen in coal [48].…”

Section: Average Scattering Length Density Of Multicomponent Samplesmentioning

confidence: 99%

Practical Aspects of Planning and Conducting SAS Experiments

Small-Angle Scattering From Confined and Interfacial Fluids

2016

This chapter begins with tips on writing a successful beam time proposal followed by the guidelines on the optimization of the instrument configuration that allow maximizing output of the high quality experimental data. Methods of the instrument and sample background subtraction are described along with the choice of optimal sample thickness and other parameters that permit minimizing contribution of multiple scattering. Placing SAS data on absolute scale is important as a sensitive indicator of the applicability of the structural model chosen for the SAS data analysis. This can be achieved by using absolute standards or direct beam measurements. Accurate estimation of the effective thickness of powder samples is required for placing SAS data on absolute scale. Several methods of such estimation using different physical parameters are described along with the contrast variation methods using liquid solutions of protonated and deuterium substituted solvents and pressurized gases as contrast matching media. Applying for Beam TimeThe vast majority of neutron and photon sources operate as "user facilities" that provide access to neutron research instruments for researchers from industry, universities, and government agencies. These facilities are operated with different modes of access. At SNS and HFIR, access is available via a general user program. Calls for proposals are usually issued twice a year, and beam time is allocated on a competitive basis after a peer-review process in which proposals are reviewed for scientific merit. A fraction of total available beam time (~25 % at SNS and HFIR) is allocated for scientific research conducted by instrument personnel and for instrument and sample environment upgrades, etc. A limited amount of beam time might be provided via a "rapid access" route for conducting urgent experiments or feasibility measurements. There is no access charge for researchers whose results are made available to the scientific community by being published in the open scientific literature. A fraction of beam time may be used for confidential, proprietary research, which can be performed only under full cost recovery. Most user facilities have financial assistance programs covering travel, room and board, etc. to encourage new users to acquire their first experience in neutron and x-ray scattering.SAS instruments are in great demand and are usually oversubscribed by a factor of 2-3. Because of the strong competition for beam time, developing good proposal writing skills is essential. Some tips that might help first-time users obtain beam time follow.One of the most important factors is choosing the "right" facility for a particular SAS experiment, including the type of radiation (neutrons vs. x-rays), power (brilliance), and the availability of the relevant sample environment. The higher radiation flux provided by synchrotron sources permits better statistics to be obtained in a shorter time, which could be critical for studying weakly scattering samples or for experiments where it is necessary to ...

“…The difference between the scattering of dry and contrast matched sample gives information about the open porosity. Contrast matching pores with liquids was used for investigation of both classes of porosity in different materials including polymer resins [45], char [46], porous graphitic materials [47,48], activated carbons [49,50], and coal [51][52][53][54][55][56]. Contrast matching of open pores can be also conducted using pressurized gases and supercritical fluids [57].…”

Section: Carbonaceous Materialsmentioning

confidence: 99%

Neutron and X-Ray Porosimetry

Small-Angle Scattering From Confined and Interfacial Fluids

2016

This Chapter considers principles of the neutron-and x-ray scattering-based porosimetry, which exploits contrast variation and contrast matching method. The coherent scattering from porous solids arises largely from the contrast between the empty and filled pores. A porous medium is exposed to a solvent vapor having the same neutron scattering length density or electron density as the matrix skeleton. The smallest pores are filled with a condensed liquid already at low relative pressure, followed by filling of progressively larger pores. Thus, measured as a function of the relative pressure, SAS probes sub-populations of pores of increasing average size, which is calculated using the Kelvin equation. Illustrative examples are given of how SAS can be used for structural characterization of various types of porous materials including low-dielectric constant polymer films, porous silicas, carbonaceous materials, and Nafion membranes.