Test - lowercase jpg
Kaixin micro test
Test probe P100-M3
inductance
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â—† Hysteresis Loop
The hysteresis loop occurs due to capillary condensation, where nitrogen molecules condense and fill mesopores under pressure. This process takes place on the liquid surface of an annular adsorption membrane within the pores. During desorption, the process starts from the spherical meniscus at the pore opening, leading to non-coinciding adsorption and desorption isotherms, forming a hysteresis loop. Another explanation involves the contact angle between liquid nitrogen and the material during adsorption: the advancing angle during adsorption and the receding angle during desorption cause differences in the Kelvin equation. While both factors may contribute, many prefer the first explanation as it's more intuitive (possibly referring to Xuan Ji?).
â—† Types of Hysteresis Loops
Different hysteresis loop types reflect specific pore structures. The H1 type represents uniform cylindrical pores, often mistaken for hexagonal structures. However, H1 does not confirm the exact shape—hexagonal structures are usually identified via small-angle XRD. H2 is more complex, typically associated with "ink-bottle" pores or irregularly packed particles, where desorption causes sudden gas release from narrow necks. H3 is characterized by higher adsorption at high pressures, indicating slit-shaped pores formed by flaky particle stacking. H4 also refers to slit-shaped pores but originates from layered structures rather than particle aggregation.
â—† No Hysteresis at Medium Pressure
When the adsorption amount is large at medium pressure but no hysteresis is observed, it suggests that the pores are very small. At relative pressures around 0.2–0.3, the pore radius is minimal, only a few molecular diameters. Capillary condensation occurs, making adsorption and desorption curves overlap. For example, MCM-41 with pore sizes of 2–3 nm shows no hysteresis.
â—† Mesoporous Analysis Models
The BJH model (Barrett-Joiner-Halenda) is commonly used for mesoporous analysis, based on the Kelvin equation applied to cylindrical pores. It tends to underestimate pore size. KJS (Kruk-Jaroniec-Sayari) offers higher accuracy, especially for materials like MCM-41 and SBA-15. Initially designed for ordered mesopores, KJS was later extended to cover a wider range, up to 30 nm, suitable for SBA-15 analysis.
◆ t-Plot and αs Methods
These methods analyze the entire adsorption or desorption curve. The t-plot method calculates monolayer adsorption and uses it to determine the thickness of the adsorbed layer. Specific surface area is derived from the monolayer volume. In non-porous samples, t-plot yields a straight line, while porous samples show multiple segments. The αs method, proposed by Sing, uses adsorption at a relative pressure of 0.4 instead of the monolayer. Both methods are interchangeable, with t = 0.538αs.
â—† Microporous Analysis
Microporous analysis requires higher vacuum, precise temperature control, and longer testing times—often ten to twenty times longer than regular samples. Due to the similarity between micropore size and probe molecule size, some probes may not enter the pores. The choice of analytical method depends on the sample, and literature references are often consulted. Although models can predict trends, they struggle to cover all pore sizes. NLDFT (Nonlinear Density Functional Theory) provides better results but is less commonly used in papers.