The true density is relative to the bulk density of the particle group, and refers to the actual mass of solid matter per unit volume of the material in an absolutely dense state, that is, the density after removal of internal pores or interparticle voids. The true density is the most basic physical parameter of the powder material, and it is also a parameter that must be used to determine the other physical properties such as particle distribution. The value of the true density depends on the chemical composition and purity of the material, and its value directly affects the material quality, performance, and use. It is of great significance for the determination of the true density. The concept of true density has been widely used in the evaluation of the characteristics of powders such as plastics, carbon materials, and black powders.
Calculation formula
True density formula: Ï=m/v
Where: Ï - true density
m---The quality of the sample
V - the volume of the sample
The above tests are often used in testing the true density of lithium-ion battery materials.
The significance of testing true density
1. When determining the specific surface area of ​​a solid material, such as a powder, it is necessary to calculate the true density data.
2. Many inorganic non-metallic materials are manufactured from powdered raw materials, so the true density often needs to be measured in scientific research or production.
3. In the manufacture of cement or ceramic materials, it is necessary to measure the fineness of the ball-milling slurry of the clay particles, all of which require true density data.
4, especially for cement materials, the final product is a powder, the determination of the true density of the production unit and the use of units are of great practical significance.
True density test method
The commonly used method for determining the true density is mainly gas volume method and liquid immersion method (pycnometer method). The so-called gas volumetric method is based on the law of gas conservation in a closed container observing the quality of the measured sample to determine the volume of the sample (powder), and then the final measurement of the sample's density by the sample's density. The so-called liquid immersion method is based on the Archimedes principle, the true volume of the powder is measured, and then the true density of the powder is calculated from the mass of the powder.
Immersion method (pycnometer method):
Applicable: Suitable for small cuts of powders, flakes, granules or parts of products.
Principle: The method for determining the true density of powders is based on the Archimedes principle.
Method: The powder to be tested is immersed in an infusion which is wetted but not dissolved, a vacuum is used to remove air bubbles, and a powder sample is discharged from a container of a known capacity to discharge the fluid of a known density, and the powder to be measured can be calculated. The true density.
Problems exist:
(1) Different samples need to use different infiltration liquids to prevent the problems of dissolution, reaction with materials, etc. Inorganic powders generally use organic solvents and materials that can cause water to react, such as cement, can use kerosene or xylene, etc. Organic liquid medium, etc.
(2) The infiltrating liquid must be able to easily wet the surface of the internal pores of the material. If the chosen wetting fluid is not suitable, it will not easily infiltrate the surface and affect the test results;
(3) When testing the powdery material, when the powder is completely immersed in the liquid, the air bubbles must be completely eliminated to determine the volume of the solution. At this time, it is necessary to use boiling to eliminate the air bubbles, and to use a constant temperature water bath to eliminate the influence of temperature. It's not easy to work, it's easy to go wrong, and the calculation process is easily affected by operating errors.
(4) Because there are many operation steps and there are many problems involved, different operators have different operation proficiency and different methods, which will directly affect the accuracy of the experimental results;
2. Gas volume method (helium gas method):
Applicable: Suitable for all kinds of powder, sheet, block materials, especially for porous materials.
Principle: Replace the liquid with a gas to determine the volume of the sample. The gas can participate in the very small pores and irregular irregularities on the surface of the sample, so the measured sample volume is closer to the true volume of the sample and can be used to calculate the density of the sample. The measured value is closer to the true density of the sample.
Method: The sample was placed in a true density tester, using helium as a medium, gradually pressurized to a specified value in the measurement chamber, and then the helium gas was expanded into the expansion chamber. The equilibrium pressure of the two processes was automatically recorded by the instrument. The law of conservation of mass, after calibrating the volume of the measuring chamber and the expansion chamber by means of a standard ball, the volume of the specimen is determined and the true density is calculated.
advantage:
(1) This method eliminates the possibility of immersion in the sample and has the advantage of not damaging the sample.
(2) The use of helium gas instead of the infiltrating liquid, the use of helium gas is a small molecular diameter inert gas, with the characteristics of easy diffusion, good permeability, good stability, quickly into the internal pores of the material, the material can not be measured by conventional methods Pore ​​and irregular surface depressions can be rapidly filled. The measured sample volume is closer to the true volume of the sample compared to the density bottle method, making the true density of the sample closer to the true value.
(3) The true density analyzer adopting this method will not cause problems with the material reaction, will not cause corrosion to the equipment, has a high safety factor during use, and the sample will not be contaminated and can be directly recovered, which is beneficial to valuable samples after testing Recycling. The instrument using this method is more simple to operate, with shorter test time, accurate test results and better repeatability. Currently, it has been produced by domestic manufacturers and has been incorporated into the industry standards of some industries.
Areas involving true density
True density is an important parameter for materials science and covers a wide range of fields: ceramics, catalysts, filter media, nuclear fuel, petrochemicals, soils, fertilizers, carbon black, coke, fiber, minerals, pharmaceuticals, cosmetics, cement, powder foods, drying Agents, powder metals, ion exchange resins, silica gel, alumina, titanium dioxide, solid foams, and the like.
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