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Specific Surface Area Of Diamond Abrasives

May 17,2023

1. Definition of specific surface area
The so-called specific surface area refers to the surface area of a solid substance per unit mass, usually expressed in meters ²/ G is expressed in units. The measurement of specific surface area is an important application of physical adsorption. Under certain conditions, a certain volume of gas is filled into a certain weight of diamond abrasive. Based on the measured changes in gas pressure, the number of gas molecules adsorbed on the surface of a unit mass solid is determined, and the specific surface area of the solid covered by gas molecules is calculated.
The specific surface area of diamond crystals is closely related to their particle size and crystal morphology: the larger the particle size of diamond crystals, the better their crystal shape (regular, complete, defect free, crack free, etc.), that is, the better their quality and the smaller their specific surface area; On the contrary, the finer the particle, the worse its crystal shape, that is, the poorer its quality, and the larger its specific surface area.
Diamond micropowders have different voids according to their particle size and shape. During their use, their physical and chemical process are carried out on these surfaces and voids. Therefore, the specific surface area is one of the important parameters reflecting the quality of diamond. The parameters of measuring the specific surface area are of great significance in production and scientific research.
Advanced industrial countries and renowned artificial diamond related companies abroad have listed specific surface area as a necessary inspection item for evaluating diamond quality. The relevant departments in China have also included the determination of specific surface area of diamond crystals in the detection standards.  

2. Principle of Specific Surface Area Testing
The physical adsorption method used to determine the specific surface area of diamond abrasives is an important method for detecting the particle shape, surface state, and internal quality of artificial diamond crystals.
Under low temperature conditions, physical adsorption occurs between the surface of the solid sample and the adsorbate gas due to van der Waals gravity. When the adsorption reaches equilibrium, measure the equilibrium pressure and the amount of adsorbed gas, and calculate the specific surface area of the sample based on the physical adsorption isotherm equation. The calculation formula will not be elaborated here.

3. Precautions for specific surface area testing
(1) Degassing treatment of the tested sample before measurement
Before determining the specific surface area, the gas previously adsorbed by the sample in the atmosphere must be completely removed – degassing treatment must be carried out to fully adsorb the known adsorbate gas. The selection of temperature and time for degassing treatment has a direct impact on the accuracy of the measurement results, as it affects the completeness of degassing. The degassing temperature and time should be determined based on the minimum temperature for graphitization of diamond crystals under vacuum conditions, the instrument temperature range, and a series of experimental results on different particle sizes and types of diamond samples. If the small specific surface area is measured using the Accsorb2100E physical adsorption instrument produced by Mac Instruments in the United States, the general degassing temperature is 350 ℃ and the degassing time is 240 minutes.
(2) Determination of Temperature – Selection of Liquid Nitrogen Temperature
According to the theory of physical adsorption, when the temperature of the tested sample decreases to the point where the liquefaction heat of the adsorbate gas is equal to its adsorption heat, the physical adsorption between the solid substance and the adsorbate gas is the most sufficient. If the temperature range is exceeded, the adsorbate gas molecules will reversibly desorb, and the measurement results will inevitably be low. The low temperature required for the physical adsorption of adsorbate gas molecules on diamond samples is generally achieved and controlled using the low temperature of liquid nitrogen. The calibration test of instrument standard samples and sample measurement experiments indicate that the specific surface area measurement temperature of diamond crystals, i.e. liquid nitrogen temperature, should be lower than -194 ℃.
(3) Selection of sample dosage
The sample dosage for specific surface area measurement should be moderate, as too little or too much can affect the accuracy of the measurement results. The measured results indicate that with nitrogen as the adsorbent, the total surface area of the sample is approximately 30.0m ², Or the specific surface area

3~ 15m ²/ g. Loading sample 2-10g
15~ 18 m ²/ g. Loading sample 0.5-2g
80 m ²/ Above g, load 0.2~0.3g of sample
▲The above is the reference dosage

The specific usage amount should be determined based on the testing equipment and actual situation
Therefore, the key to accurately measuring the specific surface area is:
(1) The tested sample must undergo degassing treatment before measurement;
(2) Strictly control the temperature during the measurement process of specific surface area;
(3) The selection of sample weight should be moderate.

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