JPS587942B2 - Kokuenkaseihanteihouhou - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides carbon (
The present invention relates to a method for determining the graphitizability of carbon (hereinafter referred to as low-temperature treated carbon).

Carbon that has not undergone high-temperature heat treatment is used as a raw material for graphite materials, such as electrodes for arc furnaces, electrodes for electric steel manufacturing, anodes for electrolysis,
It has many uses, including resistance heating elements, firebricks, structural materials, and more.

Graphite is produced by heat-treating amorphous carbon such as petroleum coke or pitch coke in a state where oxidation is suppressed.

For example, in the case of petroleum coke, heavy oil is carbonized to obtain coke, which is crushed, molded with a binder such as pitch added, fired, and then heat treated to produce graphite material. As the temperature is gradually increased, hydrogen, nitrogen,
As oxygen and other low-carbon compounds are liberated and reduced, the structure gradually shifts to a structure with stable bonds with a high carbon ratio.

As the heating temperature continues to rise, this carbon crystallizes and its crystal structure approaches a graphite crystal.

This is called graphitization, and the progress of graphitization is called the degree of graphitization.

The ease with which a material can be graphitized is called graphitizability.

Determination of the degree of graphitization of carbon is industrially important, for example, extremely important in controlling the graphitization process itself, predicting the quality of products when graphite is applied for specific purposes, quality control, and others.

Generally, the degree of graphitization depends on the treatment temperature, and the higher the treatment temperature, the higher the degree of graphitization.

However, treatment temperature cannot be used as an indicator of the degree of graphitization.

This is because graphite varies depending on the type of starting material, non-oxidizing atmosphere during processing, and other processing conditions.

For example, carbon obtained from starting materials containing aromatic compounds can be graphitized relatively easily, but carbon obtained by carbonizing cellulose or thermosetting resins does not maintain the molten state of low molecules during carbonization. Growth and stratification of aromatic planes are hindered, making it difficult for graphitization to proceed.

As described above, graphitization of carbon largely depends on the starting organic material, but graphitization is difficult to determine by conventional methods such as those described below unless the sample carbon is subjected to high-temperature heat treatment.

It goes without saying that the graphitizability of the raw carbon used to manufacture graphite products is industrially important, but if it can be determined at the raw carbon stage, it will be extremely important industrially. It is beneficial for

As techniques for measuring the degree of graphitization, X-ray diffraction, true density, electromagnetic properties, electrical resistance, coefficient of thermal expansion, etc. have been conventionally used, or a combination of these has been used.

As mentioned above, it is impossible to apply these measurement techniques to low-temperature treated carbon, which is a raw material for graphite materials, to predict and determine graphitization properties due to future heat treatment using normal methods.

The present invention relates to a method for determining the graphitizability of low temperature treated carbon at extremely low temperatures.

The present invention accurately determines the graphitizability of low-temperature treated carbon, so-called raw material carbon, which cannot be determined by the X-ray diffraction method, but it is nevertheless simple and quick.

The present inventor has been studying the relationship between the degree of graphitization of carbon and the magnetoresistive effect.

The rate of change in resistivity △ρ/ρ based on the magnetoresistive effect is a quantity defined as the ratio of the resistivity ρ when no magnetic field is applied to the measurement sample and the change in resistivity △ρ when a magnetic field is applied. but,
In practice, it is obtained by flowing a constant current through the sample and determining the ratio of the voltage drop V when no magnetic field is applied to the change in voltage drop ΔV when a magnetic field is applied.

Therefore, Δρ/ρ does not include any form factor, and there is no need to consider the porosity or the like specific to carbon materials.

According to the inventor's recent research results, the rate of change in resistance based on the magnetoresistive effect of low-temperature treated carbon is due to the anisotropic magnetoresistive effect that provides an index of future graphitizability at extremely low temperatures of about 2K or less. shows the rate of change in positive resistivity based on

Further, according to research by the present inventors, the rate of change in resistivity due to the magnetoresistive effect of low-temperature treated carbon is weak at temperatures of about 2K or higher.

This phenomenon was previously completely unknown.

Furthermore, the rate of change in resistivity based on the magnetoresistance effect at extremely low temperatures shows that the higher the graphitizability of the sample, the sharper the anisotropy, and the lower the graphitizability of the sample, the lower the anisotropy. It has been found that the rate of change in resistivity based on the resistive effect can be shown and that measurement is easy.

The present invention is based on the above findings. The present invention cools a small piece of a low-temperature treated carbon sample to an extremely low temperature of about 2 K or less, and measures the rate of change in resistivity based on the magnetoresistance effect while rotating the sample and a magnetic field relative to each other. A method for determining graphitizability is provided, which is characterized in that it is associated with graphitizability.

According to the present invention, the graphitizability of a carbon material can be easily and accurately determined at the raw material carbon stage before the carbon material becomes a graphite product.
There is also the advantage that measurements can be carried out quickly.

The measuring method of the present invention will be explained in detail below.

First, an overview of the measuring device will be described.

Figure 1 shows the outline of the system, where 1 is a superconducting magnet, 2 is a sample, 3 is a magnetic field strength detection element of a magnetometer, 4 is a magnetometer, and 5 is a magnetometer.
6 is a potentiometer, 6 is a DC amplifier, 7 is an XY recorder, 8 is a current source for generating a magnetic field, and 9 is a constant current source for passing a current through the sample.

Although not shown in the figure, the sample 2 is fixed on a rotating part of a sample rotating device, and is allowed to rotate through 20 degrees of freedom.

Further, the sample 2 and the rotating part are cooled to 2 K or less with liquid helium.

While a constant current is flowing through the sample 2, the voltage drop is balanced by the potentiometer 5, and this zero potential is input to the DC amplifier 6.

This zero potential is a reference potential. To keep the current flowing through the sample constant, use a mercury battery or a highly stable constant current power supply, for example.
During the measurement, a current of 10 mA is applied to the sample with an accuracy of, for example, 0.001%.

To measure the anisotropy of the rate of change in resistivity based on the magnetoresistance effect at extremely low temperatures, a magnetic field of an appropriate magnitude, for example 50 kG, is employed.

Next, to explain sample preparation, a sample is cut out from the low-temperature treated carbon material to be measured.

Even in such a sample, the current flows along the network plane of the condensed benzene ring system, so if the sample is arbitrarily cut out, the apparent resistivity inevitably increases and the reproducibility of the results deteriorates.

Therefore, it is preferable to cut out the sample so that the resistivity is minimized.

A constant magnetic field is applied to the sample, and the direction of the magnetic field in which the maximum value of △ρ/ρ is observed is M, and the direction of the magnetic field is changed in two planes that include M and are orthogonal to each other, and the minimum value in each is determined. Let m1 and m2 be the directions in which are observed.

Here, the direction close to the direction of the current in the sample is defined as m2.

Next, change the magnetic field in the M direction to m1 direction (temporarily assume T method),
and the M direction to the m2 direction (assuming the TL method) while rotating (actually, the sample is rotated).

Then generally was confirmed to be constant for the same sample.

According to research by the present inventors, the orientation status of the condensed benzene ring network plane can be determined as follows from the measurement results of the anisotropy of the rate of change in resistivity based on the magnetoresistive effect.

First, there are three ideal orientation situations of the mesh plane, and (△ρ/ρ)M and (△ρ/ρ)Tmin in each case.
, (Δρ/ρ)TLmin is as follows.

(1) When the mesh planes are arranged in parallel (△ρ/ρ) Tmin - (△ρ/ρ) TLmin
= 0 (11) When the mesh plane is uniaxially oriented (△
ρ/ρ) Tmin - (△ρ/ρ) M, (△ρ/ρ)
TLmin = 0 If the mesh plane is randomly distributed, the two anisotropy ratios (△ρ/ρ) Tmin / (△
ρ/ρ) M1 (△ρ/ρ) TLmin/(△ρ/ρ)
If M is taken, the orientation status of the network surface of the sample is (i), (i1
), (iii).

Now, to specifically describe the method of the present invention, first, sample A,
B, C, and D were prepared.

These samples were made by heat-treating four types of raw coke made from different types of heavy oil at 1100° C. for 30 minutes in a nitrogen stream, and were cut out as described above.

These samples were measured using the T and TL methods using the apparatus shown in FIG.

0 for current and voltage terminal conductors that connect directly to the sample.
．． A gold wire with a diameter of 0.08 mm was used, and the connection between the gold wire and the sample was made with gold paint.

Figures 2A-D show the measurement results.

The horizontal axis indicates the azimuth of the magnetic field with the base line being the magnetic field direction in which the rate of change in resistivity due to the magnetoresistive effect is the minimum value, and the vertical axis indicates the rate of change Δρ/ρ (%).

The measurement conditions were a measurement temperature of 1.7K and a constant magnetic field of 50 kG.

The following table (Table 1) was obtained from the measurement results shown in FIGS. 2A to 2D.

As mentioned above, even in the stage of low-temperature treated carbon, the current flows along the condensed benzene ring network plane, so the higher the anisotropy of Δρ/ρ, the more parallel the network planes are.

Graphitization is the effect of connecting these mesh surfaces into a planar shape through high-temperature heat treatment, removing defects within the connecting surfaces, and further layering the layers between the surfaces into a layer similar to that of graphite crystals. The more parallel the benzene ring networks are during the low-temperature treatment, the easier it is to graphitize.

That is, it has excellent graphitization properties.

According to the anisotropy study method described above, samples A and B are oriented so that their mesh planes are parallel, and sample C is oriented uniaxially.
It can be determined that sample D is randomly oriented.

From the resultant values of the anisotropy ratio, it is determined that the graphitizability is higher in the order of D, C, B, and A.

In order to examine the validity of this judgment result, the above samples A and B
. Measure the rate of change at a temperature of 77K and a magnetic field of 10kG.
The measurement was performed according to the method described above under the measurement conditions.

According to research by the present inventor, the rate of change in resistivity based on the magnetoresistance effect of graphite at 77K is closely related to the degree of graphitization of the sample (Japanese Journal
alof Applied Physics, Vol.
10, A4, pp416-420), and it has been experimentally shown that the rate of change is larger in samples with more advanced graphitization (the above materials and Japan Journal
al of Applied Physics,
Vol. 7, Jl6.4 pp 9 5 8-9 6
3), theoretically (JSPS Materials 1 1 7-1 3
2-c-1, Carbon, Vol 1 3, A3,
244) Shown.

The results are shown in the table below (Table 2). A', B', C' in the table below
, and represent raw materials A, B, C, and D, respectively.

According to the research conducted by the present inventor, it has been found that it is difficult to determine the difference in the degree of graphitization by X-ray diffraction when graphitization occurs to the degree of A', B', C', and D'. The degree of graphitization of A', B', C', and α was determined based on the rate of change in resistivity based on the magnetoresistive effect at 77K.

It can be seen that the degree of graphitization increases in the order of D', C', B', and A'.

Therefore, it can be seen that the graphitization properties of the low-temperature treated carbon samples A, B, C, and D based on Table 1 are extremely appropriate.

Note that the anisotropy generated in sample D' is the anisotropy after high-temperature treatment, so it is slightly different from the anisotropy of sample D.

As mentioned above, the rate of change in resistivity due to the magnetoresistive effect of low-temperature treated carbon at temperatures of 2K or higher is weak;
For comparison, FIG. 3 shows the rate of change in resistivity based on the magnetoresistive effect measured at temperatures of 42 K and 1.7 K for Sample C.

Obviously, under the measurement conditions of 4.2 K, positive and negative rates of change in resistivity due to the magnetoresistive effect are observed due to the magnetic field, and the values are very small.

Therefore, the rate of change in mixed negative and positive resistivity due to the magnetoresistive effect at a temperature at which the rate of change is measured cannot be used as an index of graphitizability.

To summarize the above, the graphitizability determination method of the present invention uses low temperature treated carbon samples in 2. The sample is cooled to an extremely low temperature below OK, and the rate of change in resistivity based on the magnetoresistive effect is measured while applying a magnetic field using various methods.The resulting values are processed appropriately to determine the graphitizability of the sample. It is used to obtain indicators.

The advantage of the method of the present invention is that it utilizes the anisotropy of the rate of change in resistivity based on the magnetoresistive effect of the low-temperature treated carbon sample at extremely low temperatures, so the measured values are easy to process, and the magnitude of the anisotropy is Since this has a high correlation with the graphitizability of graphite products made from the sample, it is possible to accurately obtain an index of the graphitizability of the sample, and there are other benefits as mentioned above. can get.

Having thus described the invention, it will be apparent to those skilled in the art that various modifications may be made within the scope of the invention.

For example, as a method for processing the measured value of the anisotropy of the rate of change of resistivity based on the magnetoresistive effect, in the above embodiment, the ratio of the measured anisotropy values was taken, but any other method may be used. You may do so.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram showing an example of an apparatus for implementing the method of the present invention, and Fig. 2 A, B, C, and D show the rate of change in resistivity based on the magnetoresistive effect of a low-temperature treated carbon sample at an extremely low temperature. The graph showing the anisotropy and FIG. 3 are 1.7K and 4. 2
It is a graph showing a comparison of the rate of change in K.

Claims (1)

[Claims] 1 Carbon heat-treated at a temperature of 900 to 1300°C to be determined (hereinafter referred to as low-temperature treated carbon sample) is cooled to an extremely low temperature of 2K or less, and magnetic fields are applied to the sample from various directions based on the magnetoresistive effect. A low-temperature treated carbon sample characterized in that an index of the graphitizability of the sample is obtained by measuring the anisotropy of the rate of change in resistivity and correlating the obtained anisotropy values. Graphitizability determination method.