Porous Material Testing

Porous material inspection mainly conducts scientific testing and analysis on the porosity, pore size distribution, specific surface area, permeability, mechanical strength and adsorption isotherms, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Porosity: The porosity is the indicator of measuring the percentage of the pore volume in the total volume of the porous material, and is the most basic structural parameter of the material. Usually determined through the Archimedes drainage method or mercury intrusion method. The precise porosity data are crucial for evaluating the adsorption, permeability, thermal insulation and mechanical properties of the material, and are the basis of the material design and applications.

Pore size distribution: The pore size distribution describes the quantity or volume proportions of the pores of the different sizes in the material. According to the IUPAC classification, the pores are divided into the micropores (<2 nm), mesopores (2-50 nm) and macropores (>50 nm). The common testing methods include the mercury intrusion method (suitable for the macropores and mesopores), gas adsorption method (suitable for the micropores and mesopores) and image analysis method. The pore sizes directly affect the adsorption selectivity, filtration efficiency and mass transfer rates of the material.

Specific surface area: The specific surface area refers to the total surface area per unit mass of the material, and is the key parameter of measuring the adsorption capacity, catalytic activity and reactivity of the material. Usually determined through the nitrogen adsorption method based on the BET theory. The high specific surface area is the core of the excellent performances of the porous materials (such as the activated carbon and molecular sieves), widely applied to the catalyst supports, adsorbents and electrode materials.

Permeability: The permeability characterizes the ability of the fluid (gas or liquid) to pass through the porous material under the pressure differences, and is the core parameter in the filtration, separation and membrane application fields. Its testing follows the Darcy's law, and the fluid fluxes under the specific pressure differences are measured through the dedicated permeability testers. This indicator is crucial for evaluating the filtration membrane efficiencies, battery separator performances and rock reservoir productivity, etc.

Mechanical strength: The mechanical strength evaluates the anti-damage ability of the porous material under the loads, including the compression strength, bending strength and wear resistance, etc. Since the high porosity is usually accompanied by the strength decrease, in the applications requiring both the load-bearing and functionality (such as the porous ceramic filters and porous metal implants), the mechanical testing such as the compression and three-point bending must be carried out, to ensure their structural integrity.

Adsorption isotherms: The adsorption isotherms describe the relationships between the amounts of the gas or vapor adsorbed by the porous material and the relative pressures at the constant temperatures. Through the analysis of the isotherm types (types I-VI), the pore size distributions, surface properties and adsorption mechanisms of the material can be inferred. The nitrogen (77K) and argon (87K) adsorption are the standard methods, indispensable for studying the adsorption properties of the materials such as the activated carbon, MOFs and zeolites.

Testing Scope

Inorganic porous materials: Including the porous ceramics (such as the alumina and silicon carbide), porous glasses, zeolite molecular sieves and aerogels, etc. The testing emphases are their high-temperature stability, corrosion resistance, precise pore sizes and acid site analysis. Widely applied in the high-temperature filtration, catalysis, adsorption separation and thermal insulation fields, with the extremely high requirements for the testing precision and tolerance.

Organic and polymer porous materials: Covering the porous polymers, resin foams, organic framework materials (such as the COFs) and porous carbon materials. The testing needs to focus on their pore size adjustability, surface chemical functional groups, thermal stability and swelling behaviors in the solvents. This type of material is widely applied in the gas storage, water treatment, drug release and chromatographic separation.

Metal porous materials: Such as the metal foams and sintered metal porous materials. In addition to the pore structures, the testing items particularly emphasize their electrical conductivity, thermal conductivity, fatigue strength and durability in the corrosive environments. Mainly applied to the sound absorption and vibration damping, heat exchangers, battery electrodes and biomedical implants.

Natural porous materials: Including the wood, rocks (such as the sandstone), bones and soils. The testing needs to consider the high heterogeneity and complexity of their structures, involving the permeability, capillary action, mineral compositions and micro-structures (such as the CT scanning). The relevant testing is of great significance for the geological exploration, building materials, agriculture and biomedical research.

Composite porous materials: Referring to the porous bodies composed of two or more materials of the different properties, such as the ceramic-metal composites and polymer-nanoparticle composite porous materials. The testing needs to comprehensively evaluate the interface bonding states between the components, the enhanced properties brought by the synergistic effects (such as the electrical and thermal conductivity and mechanical strengths), as well as the long-term stability in the different environments.

Functionalized porous materials: Referring to the porous materials endowed with the specific functions (such as the catalysis, antibacterial and magnetic response) through the surface modification or doping. The testing scope goes beyond the basic physical properties, extending to the surface chemical compositions (XPS), functional group densities, catalytic activity evaluation and the response properties to the external stimuli (light, electricity and magnetism).

Testing Methods

Gas adsorption method: The nitrogen, argon or carbon dioxide is used as the adsorbates, and the adsorption-desorption isotherms of the material are measured at the low temperatures (such as the liquid nitrogen temperature 77K). The specific surface area is calculated based on the BET model, and the mesopore and micropore size distributions are analysed based on the BJH, DFT or HK models. This method is the most widely used method of characterizing the nanometre-scale pores, with the standards including ISO 9277 and ASTM D3663.

Mercury intrusion method: Using the non-wetting characteristic of the mercury under the high pressures, the mercury is pressed into the material pores by applying the different pressures. According to the Washburn equation, the intrusion pressures are inversely proportional to the pore sizes, thereby calculating the pore size distributions, especially good at analysing the macropores and mesopores. This method has the wide testing ranges (usually from several nanometres to hundreds of micrometres), but the high pressures may damage the fragile pore structures, with the standard reference ASTM D4404.

Microscopy and image analysis method: Including the scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM), which can directly observe the surface morphologies and pore structures of the porous materials. Combined with the digital image processing software, the quantitative statistical analysis of the pore sizes, shapes and distributions can be carried out. This method is intuitive, but belongs to the local observation, and the multiple-point sampling is needed to obtain the statistical representativeness.

X-ray diffraction and small-angle scattering method: The X-ray diffraction (XRD) is used to analyse the crystal structures of the porous materials, especially important for the crystalline materials such as the zeolites and MOFs. The small-angle X-ray scattering (SAXS) can non-destructively statistically determine the sizes, shapes and distributions of the nanometre-scale pores, with the unique advantages for the characterization of the disordered porous systems (such as the aerogels), and the detectable size range is usually 1-100 nm.

Permeability testing method: According to the Darcy's law, the stable pressure differences are established on the two sides of the material, and the fluid (commonly the air or water) flow rates through the material are measured. Through the calculation, the absolute permeability or relative permeability of the material can be obtained. This method directly simulates the performance of the material in the actual filtration or separation processes, and is the key testing means of the membrane materials, filter media and core samples, with the standards such as ASTM D7263.

Archimedes drainage method: A classic method of determining the open porosity, closed porosity and bulk density. By measuring the dry weight of the material in the air, the saturated weight after the immersion in the liquids and the suspension weight in the liquids, the relevant parameters are calculated. This method has the simple equipment and low cost, but has the requirements for the sample sizes and liquid wettability, and is often used for the preliminary characterization of the materials such as the ceramics and metal foams.

Testing Standards

Domestic Standards:

1. GB/T 19587, Determination of the specific surface area of solids by gas adsorption using the BET method

2. GB/T 21650.1, Pore size distribution and porosity of solid materials by mercury porosimetry and gas adsorption — Part 1: Mercury porosimetry

3. GB/T 21650.2, Pore size distribution and porosity of solid materials by mercury porosimetry and gas adsorption — Part 2: Analysis of mesopores and macropores by gas adsorption

International Standards:

1. ISO 9277, Determination of the specific surface area of solids by gas adsorption — BET method

2. ASTM D3663, Standard test method for surface area of catalysts and catalyst carriers

3. ASTM D4404, Standard test method for determination of pore volume and pore volume distribution of soil and rock by mercury intrusion porosimetry

4. ASTM D7263, Standard test methods for laboratory determination of density (unit weight) of soil specimens

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Specific surface area and pore analyser: This is the core equipment of the porous material characterization, usually integrating the static volumetric gas adsorption systems, equipped with the high-precision pressure transducers and multi-station analysis ports. The mainstream equipment can carry out the fully automatic adsorption-desorption testing at the liquid nitrogen, liquid argon or ice-water temperatures, and has the built-in multiple analysis models (BET, BJH and DFT, etc.) for calculating the specific surface areas and pore size distributions, meeting the requirements of the standards such as ISO and ASTM.

Mercury intrusion porosimeter: Composed of the high-pressure chambers, pressure control systems, mercury intrusion volume metering systems and safety protection units. The modern fully automatic mercury intrusion porosimeters can programmatically raise the pressures to the maximum of about 60000 psi, automatically record the relationships between the intrusion volumes and pressures, and generate the detailed reports such as the pore size distributions, pore volumes and porosities. Its high-pressure characteristics require that the samples must have a certain mechanical strength.

Scanning electron microscope: The SEM scans the sample surfaces through the focused electron beams, exciting the signals such as the secondary electrons and backscattered electrons for the imaging. The SEMs equipped with the field emission guns have the nanometre-level resolutions and can present the three-dimensional morphologies, pore wall structures and connectivity of the porous materials. The environmental scanning electron microscopes (ESEM) can even directly observe the moisture-containing samples, avoiding the damage of the routine sample preparation to the porous structures.

Permeability tester: Specially used to measure the gas or liquid permeability of the sheet, tubular or block porous materials. The equipment usually includes the precision gas or liquid sources, pressure stabilizing systems, flow measurement units (such as the mass flow meters and soap film flow meters) and data acquisition systems. The advanced models can programmatically control the pressure differences and testing media, and automatically calculate the Darcy permeability and non-Darcy flow coefficients.

Small-angle X-ray scattering instrument: The SAXS equipment is composed of the high-intensity X-ray sources (rotating anodes or synchrotron radiation), precision collimation systems, sample chambers and two-dimensional detectors. It measures the scattering intensities of the X-rays by the sample at the extremely low angles (usually 0.1-10°), inverting the electron density fluctuation information at the nanometre scales inside the material, thereby non-destructively obtaining the statistical information of the pore structures.

True density analyser: Usually adopting the gas displacement method (commonly the helium). Using the principle that the small helium molecules can penetrate into most open pores, the skeleton volumes of the samples are precisely measured, thereby calculating the true densities. Combined with the bulk densities obtained by the Archimedes method, the open porosities of the material can be accurately calculated. This equipment is the key instrument of measuring the basic physical properties of the material density and porosity.