The surface area per unit weight of the solid porous material (i.e., specific surface area) is an important physical parameter. The real surface includes irregular surfaces and internal surfaces of the holes. Their area cannot be calculated from particle size information, but can be determined by adsorbing an inactive or inert gas at the atomic level. The amount of gas adsorbed is not only a function of the total amount of exposed surface, but also (i)? temperature, (ii)? gas pressure, and ?(iii)? a function of the intensity of the reaction between the gas and the solid. Because most of the interaction between gas and solid is weak, in order to cause it to adsorb fairly enough to cover the entire surface, the surface must be sufficiently cooled to the boiling temperature of the gas. As the gas pressure increases, the amount of surface adsorption increases in a non-linear manner. However, when the gas completely covers the surface with one atomic thickness (monolayer gas), the adsorption of the cold gas does not stop! As the relative pressure increases, the excess gas is adsorbed to form a "multi-molecular layer", and further It may be further liquefied to fill the entire tunnel.
In order to achieve the above objectives, the sample is first vacuum degassed to clean the surface of the sample; if nitrogen is used as the molecular probe (ruler), the sample is then weighed together with the sample tube and placed in liquid nitrogen (-273 ° C). The controlled amount of nitrogen that has been metered by the pressure sensor is controlled to record the amount of adsorption of the sample. The process is quite complicated and lengthy. After obtaining the data of the saturated adsorption amount of the sample under different pressures, the desired result is calculated by an empirical formula (model) determined by the nature of the sample.
To make an incomplete analogy: to measure the area of ​​a room, but in addition to many basketballs do not have the right ruler, and the diameter and cross-sectional area of ​​the basketball is known. So, before measuring the area of ​​the house, first move the furniture placed in the house, then throw the basketball into the room. The number of thrown in can be controlled and calculated. When the basketball is covered with the house, we will basketball. Multiply the cross-sectional area by the number of basketballs thrown in to estimate the area of ​​the room. In the same way, then throwing basketball until the room is filled with basketball until the roof, we can infer the size of the room. The physical adsorber is designed to achieve this entire process.
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