FR2682370A1 - Flat micronic graphite, process for its preparation and its applications - Google Patents

Abstract

Micrometric graphite particles, their preparation method and use are described. This graphite material is characterized in that it essentially consists of single crystal particles with a particle-form factor of 50-1000, which have, when deposited in a layer a few millimetres thick on a plane support, a mosaic structure in which the angular carbon plane misalignment is +/- 15 DEG , as determined by X-ray diffraction. The flat micrometric graphite is useful as an electrode material in rechargeable or non-rechargeable batteries.

Description

 The present invention relates to micron graphite particles, a process for their preparation, and their applications.

 Various pulverulent carbonaceous materials, graphitized or not, are known. They are prepared industrially by processes using different technologies.

 The carbon blacks are obtained by cracking hydrocarbons in the vapor phase. They are in the form of nanometric powder whose particles are agglomerated or not. Some of these materials may be graphed by subsequent heat treatment.

 Natural graphites are obtained after washing the ore and purifying the graphite.

 Artificial graphites are obtained by a controlled treatment of precursors (pyrolysis of pitch of coal or oil), followed by graphitation. The resulting products are then either dry milled in ball mills or milled wet in ball mills. Each product is marked by its particle size. We can thus distinguish large flakes (diameter between 0.2 and 2 mm), small flakes (diameter less than 0.35 mm), fine particles (diameter less than 112 μm), very fine particles (smaller diameter at 20 ssm).

For many applications of the lubrication type, it is sought to respect the sheet structure, that is to say, an ultra-thin crystalline structure. But conventional grinding techniques break grains in all directions
An object of the present invention is to provide a graphitized material in the form of flat micron graphite particles, characterized by a high diameter / thickness ratio.

 The invention also relates to a process for obtaining flat micron particles from graphitizable or partially graphitized carbons.

 Finally, the subject of the invention is applications of said material.

 The graphitized material according to the present invention is characterized in that it consists essentially of monocrystalline particles having a form factor of between 50 and 1000 and which, after deposition on a plane layer support of a few millimeters thick, exhibit a mosaicite, determined by X-ray diffraction, of the order of +15 °.

 Preferably, the particle form factor is greater than 100.

 The form factor represents the ratio of the diameter b of the particles to their thickness d.

 The graphite material of the present invention is obtained by a two-step process. In a first step, an expanded material is prepared from a graphitized or graphitized precursor material. In a second step, the expanded material is subjected to grinding. The grinding results from the alternating action of an ultrasound probe and a rotary mill with shear and cavitation effect.

The alternation of these two types of grinding is particularly effective for fragmenting the pre-milled expanded graphite particles into unit crystallites. Indeed, the combination of ultrasound and rotary mill produces fine particles having a very high form factor, of the order of 100 to 200. The duration of grinding sets the maximum particle size.

As an example of a shearing rotary mill, mention may be made of the Ultra Turrax crushers from the Janke Company and
Kunkel.

 Preferably, the expanded material is subjected to a pre-grinding which is essentially intended to wet the expanded graphite with a suitable liquid. Any liquid wetting the graphite can be used. By way of example, mention may be made of benzene, toluene or cyclohexane, benzene and toluene being particularly effective. Preferably, mixtures containing from 1 to 60 g / l of expanded graphite are used. The mixture of expanded graphite and wetting liquid is pre-milled, for example by a helical turbine. This pre-crushing phase makes it possible to homogenize the mixture and causes significant fragmentation of the expanded graphite flakes.

 Post-milling drying, which is intended to remove the wetting liquid, is useful for preventing particles from clumping together. The wetting liquid must be removed in order to avoid any recompaction of the flat micron graphite particles. This removal can advantageously be carried out by lyophilization, preferably when benzene or cyclohexane is used as wetting liquid.

As a precursor material, any graphite or graphitizable material in which an element can be inserted can be used. By way of example, mention may be made of natural Madagascar graphite or Brazilian natural graphite. The graphite of
Madagascar is in the form of quasi-monocrystalline particles having an average diameter of 0.5 mm and an average thickness of a few tenths of a mm. It contains less than 0.5% ash. The graphite of Brazil is in the form of quasi-monocrystalline particles having an average diameter of 40 μm and a thickness of a few tens of ym to a few hundred of Am. One can also cite graphite or graphitizable pitch cokes.

 The precursor material may be foamed by a method of interposing a reagent into the precursor material, and then subjecting the resulting insertion compound, optionally washed with water and dried, to a sudden rise in temperature (thermal flash). The rapid start of the intercalate is accompanied by an irreversible one-dimensional expansion (or exfoliation) of each graphite particle. As reagent, it is possible to use, for example, ferric chloride-ammonia mixtures or sulfuric acid. However, the reagents having an exothermic decomposition, such as those described in the patent application FR 91.97516, are particularly advantageous. Among these reagents having an exothermic decomposition, there may be mentioned perchloric acid, used alone or in a mixture with nitric acid, or else nitromethane. The apparent density of the expanded graphite thus obtained is very low (a few grams per dm3 against 2 kg per dm3 for the precursor material). Its specific surface is of the order of a few tens of m2 / g, whereas it is of the order of 1 m2 / g for the precursor material.

 The present invention is described in more detail in the following examples, given by way of illustration but not limitation, and which relate to the preparation of the graphite material of the present invention, from different graphitized or graphitizable materials.

 The particle size distribution histogram was established by counting and determining the particle size using a light scattering apparatus, for example of the Malvern type manufactured by Instrumat which implements the scattering of a particle. laser beam by particles suspended in a carrier liquid. The histogram is spread between 0 and 100 clam. It depends on the grinding time in the first four hours and the wetting power of the liquid used. It gives the main dimension of the particles.

 The morphology of the product obtained is determined by scanning electron microscopy. The particles are deposited on a previously polished brass support. The accelerating voltage of the incident electron beam is 21 KV. The thickness of the particles is such that the stripes of the support are visible by transparency, as well as the nodules of the copper in the brass matrix. This phenomenon, linked to the existence of backscattered electrons, reveals particles whose thickness is less than 0.1 μm. This electron microscopic examination gives results point by point.

 The specific surface area of the flat micron graphite obtained was evaluated by exploiting the 77 k Krypton adsorption isotherms. The qualitative analysis of the isotherms shows that the product is characterized by a surface homogeneity identical to that encountered with a graphite natural. The quantitative exploitation of the isotherms makes it possible to determine the specific surface area of the flat micron graphite. Given the sheet morphology of flat micron graphite particles, the hiding power is deduced therefrom.

 The crystallinity of the material is determined by electron diffraction. The particles are deposited by filtration on a nucleopore membrane before being recovered on an observation grid. The small thickness of the particles makes it possible to achieve the diffraction conditions giving the maximum resolution and resulting in a monopunctuated diffraction pattern comprising the reflections [hko of the graphite.

 The depositability of flat micron graphite particles was assessed on a layer of particles deposited by settling on a thin beryllium plate.

The X-ray diffractometric recording, carried out in transmission, shows the hko reflections, but also the reflection 002, whose presence and intensity allow a good appreciation of the disorientation state of the particles.

EXAMPLE 1
A natural graphite, UF4, supplied by Carbone Lorraine and having a particle size distribution histogram compatible with an average diameter of 6.4 mA was used.

 1 g of this material was contacted with 4 ml of a mixture of 70% commercial perchloric acid and fuming nitric acid (50/50 volume) for 2 hours.

After reaction, this heterogeneous medium was filtered under vacuum. The cake recovered after filtration was placed in the bottom of a reactor previously heated to red. This first step led to the expansion of the particles of the starting material. The expanded particles were then comminuted for about 1/4 h with a helical turbine at 2000 rpm. Then they were milled for 3 hours, said grinding consisting of alternating 10 min grinding sequences using an Ultra-Turrax rotary mill and sequences of 5 minutes of ultrasound. For this grinding step, the particles were wetted with cyclohexane.

 The product obtained has a distribution histogram compatible with an average diameter of 6.2 mp. The observation of the particles with a scanning electron microscope shows a morphological state very different from that of the material before grinding. After treatment, the particles are transparent under the impact of the electron beam. The diameter / thickness ratio has therefore greatly increased after grinding.

EXAMPLE 2
The procedure of Example 1 was reproduced with a non-graphitized pitch coke, obtained by carbonization at 11000C of a coal pitch. This non-graphitized pitch coke had a histogram compatible with an average diameter of 50 m.

 The product obtained has a distribution histogram compatible with an average diameter of 28 m.

EXECUTIVE 3
The procedure of Example 1 was repeated with a graphite pitch coke obtained by heat treatment at 2500 ° C of the non-graphitized pitch coke used in Example 2.

This graphitized pitch coke had a histogram compatible with an average diameter of 50 μm.

 The product obtained has a distribution histogram compatible with an average diameter of 12 my.

 These results show that the process of the present invention is effective for cokes, and more particularly for coke graphites, thus easier to insert.

EXAMPLE 4
An insertion compound was prepared from Madagascar graphite and sulfuric acid.

 The resulting insertion compound was washed with water and dried at 110 ° C. A sharp rise in temperature to 1000 ° C. caused expansion of the graphite.

Various mixtures containing from 1 to 60 grams of expanded graphite in 1 liter of benzene were comminuted for about 1/4 hour using a helical turbine at 2,000 rpm. Then, each mixture was subjected alternately to an ultrasonic probe (20KHz) and to a grinder of the type
Ultra Turrax (20,000 rpm). Each sequence lasted 15 minutes, including 5 minutes of ultrasounds and 10 minutes of Ultra Turrax. The reactor was thermostated to prevent evaporation of the carrier liquid.

 The benzene was removed by solidification of the mixture at 0 ° C. and dynamic pumping of the vapors with trapping of these at 180 ° C. After this last treatment, the flat micron graphite is obtained in the dry state.

 The size distribution of the agitated particles in a carrier liquid was determined using a Malvern type apparatus. Figure 1 gives the evolution of the size of the particles as a function of the grinding time for a mixture at 1 g / l of graphite. It is noted that all the particles have a diameter of less than 30 μm.

 The particles obtained have a thickness of less than 0.1 a specific surface area of the order of 20 m 2 / g, and consequently a covering power of 10 m 2 / g.

 The electron diffraction pattern of the resulting flat micron graphite indicates almost perfect monocrystallinity of the material.

 A layer of a few tenths of a millimeter thick, which results from the superposition of several thousand elementary particles, is characterized by a disorientation of the carbon planes of + angle. This result reflects the excellent ability of the material to lay flat.

EXAMPLE 5
An insertion compound was prepared from Brazilian graphite and a mixture of nitric acid and perchloric acid water, according to the procedure described in the patent application.
FR 91.97516.

 The resulting insertion compound was washed with water and dried at 110 ° C. A sudden rise in temperature to 600 ° C. caused the expansion of the graphite.

Various mixtures containing from 1 to 60 grams of expanded graphite in 1 liter of benzene were comminuted for about 1/4 hour using a helical turbine at 2,000 rpm. Then, each mixture was subjected alternately to an ultrasonic probe (20KHz) and to a grinder of the type
Ultra Turrax (2,000 rpm). Each sequence lasted 15 minutes, including 5 minutes of ultrasounds and 10 minutes of Ultra Turrax.

 The benzene was removed by solidification of the mixture at 0 ° C. and dynamic pumping of the vapors with trapping of these at 1800 ° C. After this last treatment, the flat micron graphite is obtained in the dry state.

The size distribution of the agitated particles in a carrier liquid was determined using a Malvern type apparatus. Figures 2 and 3 respectively show the evolution of the particle size as a function of the grinding time for a mixture of 1 g / l and 10 g / l of graphite. It is noted that all the particles have a diameter of less than 30 μm and that 33% of the particles have a diameter of between 4 and 14 μm.
FIG. 4a and FIG. 4b respectively represent the particle size distribution histogram of the product obtained from a 10 g / l mixture milled for 5 h and for 11 h.

 The particles obtained have a thickness of less than 0.05 m and a specific surface area of the order of 40 m 2 / g, and consequently a covering power of 20 m 2 / g.

 The electron diffraction pattern of the resulting flat micron graphite indicates almost perfect monocrystallinity of the material.

The GMP flat micron graphite of the present invention can be advantageously used as electrode material in batteries, rechargeable or otherwise. For this application, the flat micron graphite is used in the form of a mixture with MONO 2 or with any other material whose electrochemical and / or morphological properties are similar to those of MnO 2, for example V 2 O 5. In fact, a percolation phenomenon occurs in GMP-MnO2 mixtures, even for
GMP of the order of 1%. Example 6 below illustrates this property.

EXTENSION 6
r
A mixture of GMP and MnO 2 is homogenized after being wetted with cyclohexane. The homogenization is carried out using an ultrasound probe with a power of 500 watts in a volume of 100 cm3 with a frequency of 20 KHz for 10 min. The cyclohexane is then extracted from the mixture by lyophilization. The recovered powder is pelletized at a pressure of 1000 kg / cm 2. The measurement of the electrical conductivity is performed on the pellet obtained by the method of Van Der Pauw. According to this method, silver-lacquer contacts as fine as possible at the four vertices A, B, C and D of a rectangle are produced on the above-mentioned tablet. In a first step, a weak current, of the order of pA or even nA, is circulated from A to B, by noting the value of the voltage appearing between C and D for different current intensities. The curve representing the voltage as a function of the intensity of the current is a line having a slope R1. Then, the operation is repeated by passing a current from D to A and the voltage occurring between the points B and C is noted. The line represents the variation of the voltage between B and C as a function of the intensity of the current. a slope R2. The resistivity p of the layer formed by the pellet is given by the following formula P = 2ion2 (R 1 + R 2) f (RR 2 1 in which e represents the thickness of the layer, R 1 and R 2 the resistances obtained by the voltage measurements and of current and f (R1 / R2) represents the van der Pauw function which is tabulated.

The results of the measurements are collated in the following table.

BOARD

<tb>% <SEP> volume <SEP> of <SEP> GMP <SEP> Thickness <SEP> of <SEP><SEP> Resistivity <SEP> (name) <SEP>
<tb><SEP> tablet <SEP> (in <SEP> cm)
<tb><SEP> 10 <SEP> 0.67 <SEP> 0.022
<tb><SEP> 6 <SEP> 1.35 <SEP> 0.064
<tb><SEP> 3 <SEP> 1.65 <SEP> 0.280
<tb><SEP> 1 <SEP> 0.48 <SEP> 4.47
<Tb>
These results show that the percolation phenomenon is reached in these biphasic media with a GMP content as low as 1%.

In conventional stacks, graphitized thermal black particles are spheroidal and the percolation threshold is only obtained with black percentages close to 30%.

 All of the properties of the flat micron graphite particles of the present invention allow their use for many applications. Thus, the particles of the present invention can be used in conventional tribology. They can also be used as an insertion compound with an inserted element suitable for vacuum tribology or high temperature applications.

They can also be used as such, mixed with pitch, to promote the graphitization pitch by heat treatment.

Claims (9)

 1. Graphite material characterized in that it consists essentially of monocrystalline particles having a form factor of between 50 and 1000 and having, after deposition layer of a few millimeters thick on a flat support, a mosaicite determined by diffraction of X-rays, of the order of +15 degrees.  2. Graphite material according to claim 1, characterized in that the form factor is greater than 100.  3. Process for producing a graphitized material according to claim 1, characterized in that it consists in preparing an expanded graphite from a graphitized or graphitized precursor material and then grinding the graphite. obtained foaming, said grinding being performed by the alternating action of an ultrasound probe and a shearing rotary mill.  4. Method according to claim 3, characterized in that the precursor material is expanded by inserting an interlayer in said material, then subjecting the resulting insertion compound to a sudden rise in temperature.  5. Method according to claim 3, characterized in that the expanded graphite is pre-milled in a wetting liquid.  6. Method according to claim 3, characterized in that the wetting liquid is removed after grinding.  7. Method according to claim 6, characterized in that the wetting liquid is removed by lyophilization.  8. Application of the graphitized material according to any one of claims 1 to 2, to the preparation of electrode materials.  9. Application of the graphite material according to any one of claims 1 to 2, graphitization of pitch.