How to detect the purity of graphite material

To understand the main test methods for the purity of graphite materials, we must first clarify several concepts:
Purity: The purity of graphite is the mass fraction of carbon in the graphite, usually expressed as 99%, 99.9%, or several 9.
Ash: Residues of graphite, usually expressed in% form, after burning at high temperatures to completely escape graphite and volatiles. There is a big gap between this concept and the reality, for example, graphite contains S element, when burned into sulfur dioxide gas volatilization, ash content data does not reflect the presence of sulfur this element. In addition, for example, iron elements in graphite in the form of elementary substances, after burning into ferric oxide, weight increases, so the ash content data in the statistics of such elements is too large. So ash is only a reference data, and there is a certain gap between the actual impurity content, and the gap level is related to the content of elements in graphite.
Impurity content: The content of a non-carbon element in graphite, usually when it comes to the impurity content of an element, or the total impurity content. Usually it is expressed by ppm, that is, the mass ratio of impurity elements in graphite is one millionth of a unit. This concept is opposite to the concept of purity. It is also a concept that many people often say about the purity of ppm.
Dust emission: Japanese Ishimoto talked about this concept when giving a lecture, that is, graphite placed in an airtight container, heated to a certain temperature, one end into high-purity nitrogen, and then at the other end of the collection of gas, testing the content of elements in the gas. Dust emission is different from other concepts. Firstly, there is no corresponding relationship between dust emission and impurity content in graphite. Elements in graphite with high impurity content in graphite do not necessarily evaporate at service temperature, and if they do not evaporate, they will not affect the products in the furnace; secondly, carbon is also a component of dust emission. Under the conditions of use, carbon can be volatilized or the amount of dust can be increased. This concept is the closest to the user’s experience, but testing is difficult and requires special devices. This concept has not been put forward by other units except KUREHA.
About testing methods
Several methods have been used, such as plasma emission spectrometry (ICP), high resolution glow discharge mass spectrometry (GDMS) and combustion method.
1. plasma emission spectroscopy (ICP)
Plasma emission spectroscopy is currently the most accurate means to detect trace elements, some elements can reach ppb level, detection limit is relatively low, and the detection limit of each element is different. The main principle is to pass the liquid sample through the tube into the test room, bombarding the sample with high-purity plasma, so that the element energy transition, different elements of the transition will produce different spectra, high-precision detector analysis of different spectral intensity to indirectly calibrate the content of each element, this method is also called ICP-AES; if according to the elements; Mass spectrometry is used to calibrate the content of each element. This method is also called ICP-MS. The accuracy and reliability of ICP-MS test is better than that of ICP-AES. The ICP method can test most of the elements except helium, neon, argon, krypton, xenon and other inert gases. According to the testing principle, the result of this method corresponds to the concept of impurity content mentioned above.
Advantages:1) high detection accuracy, can detect most of the impurity elements, the detection result is impurity content.
Disadvantages: 1) difficulty in sample preparation. ICP method requires that the test sample must be liquid sample. Because the carbon material is acid-base resistant and corrosion-resistant, it is difficult to dissolve in a certain solution, so the difficulty of sample preparation becomes the bottleneck restricting the purity of graphite material by ICP method. A previous test in Shimadzu, Japan, used microwave digestion of carbon felt samples in strong acids at high temperatures and pressures to dissolve graphite trace amounts into strong acids, and then ICP testing. Shimadzu adopted the method of doing several different concentrations of solution, because the concentration and impurity content have a linear relationship, in order to remove the impurity background of the solution and normalize the data. Although this method is technically possible, few units have been found with both microwave digestion and ICP testers, and the sample preparation process requires very specialized operation.
2) high price. At present, the price of four elements for ICP test is 80 yuan, and then every additional element is increased by 5 yuan, so the test is about 200 yuan one time. If you want to test different concentrations, you need to do at least five samples, so that is 1000 yuan, plus the cost of sample preparation, the cost will be around 1500 yuan.
2. high resolution glow discharge mass spectrometry (GDMS).
The principle of high-resolution glow discharge mass spectrometry is to apply thousands of degrees of high temperature on the surface of graphite material to make the surface elements evaporate instantaneously, and the element’s extranuclear electron energy is stimulated to emit spectra, and then to calibrate the content of each element by analysis. This test method also tests impurity content, which can test all elements except C, H, O and N. The test cycle is about 7 working days. The detection limit of elements is about 0.01ppm.
Advantages: 1) simple preparation. High resolution glow discharge mass spectrometry can be used to test solid samples, that is, the graphite blocks can be directly tested. The pollution caused by sample preparation and other processes is avoided.
2). High accuracy. Because the effect of sample preparation on material is reduced, the accuracy of test is high.
Disadvantages: 1) the price is high. The cost of each sample is 4200 yuan and 25 yuan 3500 yuan.
3. combustion method
This method tests the ash content, and the testing principle is consistent with the definition of ash, that is, calculating the ash content by the ratio of the burned graphite residue to the original mass of graphite. This test method is a rough statistical method, for the volatile elements after burning can not be statistical, for the quality of the elements after burning larger statistical deviation, so ash test can only be used as a rough reference, can not be used as a basis for impurity content. But at present, the testing methods of graphite purity in China are basically combustion methods.
Advantages: 1) simple test. Testing equipment and testing principles are relatively simple, there are electronic balance, crucible, muffle furnace can test ash.
2) customers are more recognized. At present, most of the gray fraction data that customers are exposed to, although the customer misunderstanding of this data, but can be recognized by customers.
Disadvantages: 1) less test basis. Although there are several ash test standards in China, the details of the implementation affecting the test accuracy are not described, and the standards are very similar. Ash measurement accuracy is hundreds or thousands of ppm.
2). The reproducibility is poor. Because the ash test is a relatively rough test method, the two test results may be quite different.
About burning ash test
At present, there is no ash test standard for our high-purity isostatic pressing graphite products, so we can only draw lessons from the existing standards, taking this as the lowest standard, according to our material characteristics were refined. The national standard for ash testing errors are basically hundreds of ppm, for our purity of several ppm or dozens of PPM graphite material is obviously not applicable. According to the national standard “Graphite Chemical Analysis Method” (GB/T 3521-2008), there are many domestic standards for carbon or graphite ash, and they repeat each other. Through understanding with the National Graphite Product Supervision and Inspection Center, they are based on the domestic standards, and make corresponding adjustments to the ash test. Strictly speaking, domestic testing standards are not suitable for the materials we test. We can only select a national standard as the framework, by determining the implementation rules, as far as possible to reduce the ash test results in a smaller range of fluctuations, and maximize the reproducibility of the results.
The question of purity testing depends on what data the customer needs. If you are simply studying the effect of non-carbon elements on the product during the use of carbon-graphite materials, it is recommended to use the concept of dust emission, specific test methods need to communicate with customers and then test. If only ash, it is recommended to use national standards, the National Graphite Materials Supervision and Testing Center to do.
Author: Zhao Wei