GB/T 24490-2009Test method for purity of multi-walled carbon nanotubes (English PDF)

4.1 General

This method is applicable to the determination of the ash content (mass fraction) of multi-walled carbon nanotube samples. Fully oxidize a certain amount of sample, in a high-temperature air atmosphere at 900 °C, until the carbon in the sample overflows in the form of gaseous oxides. Measure the mass of ash. Then calculate the ash content (mass fraction).

4.2 Instruments

a) Crucible with a cover: It is made of platinum, quartz or other materials, that do not change under the measurement conditions, with a capacity of 50 mL ~ 100 mL;

b) Desiccator: It is equipped with an effective and sufficient desiccant and a porous metal plate or porcelain plate;

c) Muffle furnace: There is a device for controlling and adjusting the temperature, which can provide an incineration temperature of 900 °C;

d) Analytical balance: The accuracy is 0.1 mg.

4.3 Analytical procedures

a) Sample pretreatment: Mix the sample thoroughly. Place it in a muffle furnace.

Keep it warm at 120 °C for 5 hours. Then transfer it to a desiccator. Cool it to room temperature for storage.

b) Pretreatment of the crucible: First use diluted hydrochloric acid to wash the crucible. Then use tap water to wash it. Use deionized water to rinse it. Place the cleaned crucible in a muffle furnace. Heat it at 900 °C for 30 min. Take it out and put it in a desiccator, to cool to room temperature. Then weigh it, accurate to 0.1 mg.

c) Weighing of the sample: Weigh 1 g ~ 2 g of the sample, accurate to 0.1 mg. Place the sample evenly in the crucible, without compacting it.

d) Charcoal burning: Cover the crucible and put it into the muffle furnace. Keep the air atmosphere of natural convection in the furnace. Raise the temperature to 900 °C. Keep this temperature, until all the remaining carbon is oxidized and overflows, which generally takes 3 h ~ 5 h. Place the crucible and the residue in it in a desiccator, to cool to room temperature. Weigh it, accurate it to 0.1 mg.

4.4 Results presentation method

The ash content wh can be obtained from formula (1):

Where: wh - Ash content (mass fraction); n1 - The mass of the crucible with a lid, in grams (g); n2 - The mass of the crucible and the sample before ashing, in grams (g);

Where: wh - Ash content (mass fraction); wC - Carbon phase content (mass fraction); m0 - The initial mass of the sample, in milligrams (mg); m300 - The mass of the sample at 300 °C, in milligrams (mg); m850 - The mass of the sample at 850 °C, in milligrams (mg); m900 - The mass of the sample at 900 °C, in milligrams (mg).

For one sample, a set of TGA measurements are performed three times independently.

Ash content (wh1, wh2, wh3) and carbon phase content (wC1, wC2, wC3) are calculated according to formula (4) and formula (5), respectively.

Calculate the average value and average variance of the ash content, which is measured by three TGAs; the calculation method is as shown in formula (2) and formula (3). In the ash content measured three times, if there is a result with a variance greater than 2 times the average variance, it shall be considered as an abnormal result, which is caused by sample inhomogeneity or measurement problems AND shall be eliminated and re-measured; then recalculate the average value and average variance.

The average ash content, which is obtained by TGA, is compared with the average ash content, which is obtained by burning charcoal. If the absolute deviation is greater than 1%, an additional set of TGA measurements (three times) is required.

After judging that the absolute deviation of the ash content, which is measured by TGA and charcoal, meets the requirements, take one or two sets (if there are two sets for TGA detection, two sets shall be selected) of TGA results, to calculate the average value of carbon phase content and the average variance sigma C2. Taking a set of TGA as an example, the calculation method is as shown in formula (6) and formula (7): […]

6.1 General

Through the observation and image analysis of the transmission electron microscope, the type and proportion of the carbon phase in the sample are determined.

In TEM observation, the carbon phases other than multi-walled carbon nanotubes are regarded as carbon phase impurities. According to the provisions of GB/T 24491: "When the transmission electron microscope is magnified more than 100000 times, it is observed as fibrous, meanwhile the ratio of length to diameter is greater than 20." Therefore, if the ratio of length to diameter is less than 20, it is deemed as carbon phase impurities. When the carbon phase impurities (such as graphite flakes, etc.) are large in size and cannot be clearly identified in the same field of view, as the multi-walled carbon nanotubes, the measurement method of this standard is not applicable.

If any 10 fields of view on a microgrid contain only multi-walled carbon nanotubes and no other carbon phase impurities (as shown in Figure 3a), it can be determined that the carbon phase content, which is obtained by TGA measurement, is all corresponding to the multi-walled carbon nanotubes. At this time, the measured by TGA is the content (mass fraction) of multi-walled carbon nanotubes.

If obvious carbon phase impurities are observed by TEM (as shown in Figure 3b), it shall increase the number of TEM sample preparation and acquired TEM images; meanwhile carry out quantitative analysis of TEM images. For a multi-walled carbon nanotube product, it shall carry out at least two independent sampling, dispersion, sample preparation (at least two micro-grids), take at least 15 TEM images, that meet the quantitative analysis requirements, for each micro-grid.

All TEM images of a microgrid are calculated and accumulated, according to Appendix A, to obtain the total volume V1 of multi-walled carbon nanotubes and the total volume V2 of carbon phase impurities. Neglecting the density difference of different carbon phases, the proportion of multi-walled carbon nanotubes in the carbon phase YC can be obtained, by formula (8):

If the absolute deviation of the YC results, which are obtained by two microgrids, is greater than 5%, then, it is required to add at least 15 TEM images, that meet the quantitative analysis requirements for each microgrid, to perform quantitative statistics again. If the absolute deviation of the YC results, which are obtained by the two micro-grids, is still greater than 5%, then, it is judged that the uniformity of the sample is poor, so this measurement method is not applicable.

All the TEM images of each microgrid of a sample are accumulated, to obtain the total volume V1 of multi-walled carbon nanotubes and the total volume V2 of carbon phase impurities. Use the formula (8), to calculate the proportion YC of the multi-walled carbon nanotubes in carbon phase. The content (mass fraction) of multi-walled carbon nanotubes in the sample can be obtained, by formula (9): […]

A.1 Geometric model and characteristic parameters of multi-walled carbon

nanotubes Geometric model: Cylindrical tube model.

The volume V1 of multi-walled carbon nanotubes is calculated according to formula (A.1):

Where:

V1 - The volume of multi-walled carbon nanotubes, in cubic nanometers (nm3);

D1 - The outer diameter of multi-walled carbon nanotubes, in nanometers (nm);

D2 - The inner diameter of multi-walled carbon nanotubes, in nanometers (nm);

L - The length of multi-walled carbon nanotubes, in nanometers (nm); i - Any one multi-walled carbon nanotube.

Note: The sum means to accumulate all the multi-walled carbon nanotubes in one image. The same goes for the following. A.2. Geometric model and characteristic parameters of carbon phase impurities […]