Definition
Porosity measuring instrument is a special experimental testing equipment used to determine the ratio of pore volume to the total volume of materials in porous materials. It provides key data support for material property evaluation by quantifying pore structure characteristics. This instrument is widely used in materials science, geological exploration, construction engineering, environmental monitoring and energy storage, and is one of the important tools for studying the internal structure of materials.
Rationale
Porosity measurement is based on classical theories such as physical displacement and gas adsorption. Common principles include gas expansion, mercury intrusion, and liquid saturation. The gas expansion method calculates pore volume by measuring the change in pressure of a gas between a known volume chamber and a sample chamber based on Boyle's law. The mercury intrusion method is based on the law of intrusion of non-wetting liquids into pores under external pressure, and the pore size distribution and porosity are estimated by the relationship between pressure and intrusion volume. The liquid saturation method calculates the pore volume of the open pores by measuring the mass difference between the sample before and after saturating the liquid, combined with the liquid density.
The basic relationship of the gas expansion method can be expressed as: Vp = Vc (P1/P2 - 1), where Vpis the pore volume, Vcis the volume of the reference cavity, P1and P2They are the pressure values before and after equilibrium.
Measurement method
According to the difference between the measurement principle and the object, the porosity measurement method can be divided into various types. The gas adsorption method is suitable for microporous and mesoporous materials, and the specific surface area and porosity are calculated by using the BET model or DFT method by analyzing the adsorption isotherm of the gas on the surface of the material. Mercury intrusion is suitable for macroporous and partially mesoporous materials and can measure pore sizes typically ranging from a few nanometers to hundreds of microns. The water saturation method is commonly used for building materials and rock samples to calculate the porosity of the opening hole according to the Archimedes drainage principle. In addition, the micro-CT method based on image analysis can realize the three-dimensional visual measurement of pore structure, but the equipment cost and data processing complexity are high.
Influencing factors
Sample preparation is the primary factor affecting measurement accuracy. Sample size, shape, and surface cleanliness can affect the uniformity of gas adsorption or liquid penetration. For the mercury intrusion method, the setting of mercury surface tension and contact angle parameters will directly affect the pore size calculation results. Environmental conditions such as temperature and humidity may cause changes in sample volume or adsorption layer thickness. Instrument calibration status and operational specification are also important factors, as reference cavity volume calibration errors or pressure sensor drift can lead to system errors. The properties of the material itself, such as pore connectivity, shape complexity, and chemical properties, can also limit the suitability of the measurement method.
Applications
In the field of building materials, porosity measurement is used to evaluate the durability and mechanical properties of concrete, ceramics, and thermal insulation. In the geological and petroleum industries, this technique is used to analyze the permeability and storage capacity of rock reservoirs. In terms of energy materials, porosity data have guiding significance for the research and development of battery electrode materials, catalyst carriers and hydrogen storage materials. It can be used in environmental engineering to study the pore structure efficiency of filter materials and adsorbents. In addition, porosity is also one of the key parameters for characterizing material quality in food science, textile testing, and composite material research and development.
Instrument selection
When selecting a sample, the sample characteristics and measurement requirements should be clarified first. For microporous materials, a gas adsorption meter can be considered; For hard macroporous materials, mercury intrusion meters may be more suitable. The measurement range should cover the expected pore size distribution of the sample, while paying attention to the lower and upper limits of the instrument. The measurement accuracy and repeatability indicators should refer to the requirements of relevant national standards or international standards. Operational complexity and measurement cycle are also factors to consider in actual work, and some fully automatic models can improve batch inspection efficiency. In addition, the instrument should be considered for extended functions, such as whether it supports special analysis modes such as chemisorption and vapor adsorption. Equipment maintenance needs and manufacturers' technical support capabilities are also important considerations in long-term use.