Chemistry of inorganic compounds – Carbon or compound thereof – Elemental carbon
Reexamination Certificate
2000-08-07
2004-08-31
Hendrickson, Stuart (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Elemental carbon
Reexamination Certificate
active
06783747
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a thermal treatment apparatus for producing a carbon material which is employed as a filler; i.e., which is added, as a composite material, to resin in order to improve the physical properties of the resin, such as electrical conductivity and heat conductivity, or which is employed in a variety of batteries such as a lithium-ion battery which has recently become of interest; and to a thermal treatment method for producing the graphite carbon material.
BACKGROUND OF THE INVENTION
In recent years, portable electronic apparatuses of smaller size, such as cellular phones, video cameras, and notebook computers, have been developed at a remarkable pace. In accordance with this trend, there has been increasing demand for compact secondary batteries of high performance. Particularly, a lithium-ion secondary battery is suitably employed as a power source for a variety of portable electronic apparatuses, since it has high energy density and long service life. Therefore, in recent years, production of lithium-ion secondary batteries has drastically increased, and a further increase is expected in the future.
Graphite material is employed in the anode of a lithium-ion secondary battery, and thus in correspondence with an increase in demand for the battery there has been keen demand for graphite powder.
Carbon material that is easily converted to graphite (hereinafter called “carbon material of easy graphitization”) has become of interest as a material for the aforementioned battery or as a filler for a composite material, and extensive studies have been performed on a variety of such carbon materials.
For example, in order to enhance capacity characteristics of the battery, the crystallinity of graphite must be improved, and thus the carbon material must be subjected to heat treatment at 2,500° C. or more for graphitization.
Conventionally, the following two methods are available for producing a mass of graphite micropowder:
(1) a method in which material of easy graphitization is heated at high temperature, and then the graphitized material is crushed, producing graphite powder.
(2) a method in which material of easy graphitization is crushed in advance, and then the crushed material is heated at high temperature.
In method (1), carbon material of easy graphitization, such as any of a variety of cokes, is heated by means of resistance heating of filler carbon powder in powder to which electricity is supplied; i.e., the material is graphitized by means of an Acheson furnace. Alternatively, the material is graphitized by means of a heating furnace comprising a graphite heater. Subsequently, the resultant graphite is crushed, producing graphite powder. At the present time, the method is typically employed, but the method involves some drawbacks. For example, graphitized carbon is smooth (i.e., graphitized carbon is usually employed as a lubricant), and when the graphitized carbon is crushed, flakes tend to form. Thus, when such flakes are employed in an electrode, the flakes are deposited on the surface of the electrode, the surface becomes mirror-like, and the electrode has poor permeability for an electrolyte. As a result the performance of a battery comprising the electrode may deteriorate. Therefore, method (1), in which graphitized material is crushed, cannot produce graphite powder which is suitable for use in batteries and a variety of composite materials; i.e., which exhibits excellent characteristics.
In method (2), material to be treated; i.e., carbon material of easy graphitization, such as coke, is crushed in advance into powder of a suitable particle size. Subsequently, the powder is placed and sealed in a crucible made of carbon, and the crucible is placed in a furnace for graphitization, to thereby produce graphite powder.
This method is preferable from the view that coke is easily crushed as compared with graphite, and that flakes rarely form when coke is crushed as compared with when graphite is crushed.
Therefore, in method (2), graphite powder which is suitably employed in the anode of a lithium-ion secondary battery is produced. However, method (2) involves some problems with regard to heat treatment, as follows.
Carbon material to be heated assumes the form of powder, and thus the material must be placed in a heat-resistant container such as a crucible before the material is heated. Conventionally, a variety of apparatuses and methods have been available for heating the material in a container such as a crucible.
For example, as described above, a crucible containing carbon material is buried in coke powder which is placed in an Acheson furnace, electricity is supplied to the coke powder, and the carbon material in the crucible is heated by means of heat generated from the coke, to thereby graphitize the carbon material. However, this method is of batch-type, and thus a prolonged period of time is involved in carrying out a cycle involving elevating the furnace temperature, heating at a predetermined temperature, and lowering the furnace temperature. In addition, placing the coke powder in the furnace is troublesome, as is removing the powder after completion of heat treatment. Therefore, productivity is considerably poor and the process is unsuitable for mass production.
Furthermore, there is a possibility that the carbon material to be heated will be contaminated with gas of sulfur or metal which is generated from the filler powder and migrates into the carbon material. Such contamination caused by migration of gas may deteriorate the characteristics of graphite carbon powder and may impair battery characteristics.
The temperature within the furnace may vary greatly from position to position in the furnace in accordance with the packing density of the filler powder. Therefore, crucibles containing the carbon material must be placed in the furnace such that the temperatures of the crucibles become as uniform as possible, and thus control of the crucibles may be troublesome. In addition, in order to make the temperatures of the crucibles uniform, the crucibles must be heated for a relatively long time. As a result, carbon material particles tend to stick to one another, and therefore require re-crushing.
A high-frequency induction furnace or a resistance furnace comprising a heater have also been used. These furnaces are provided with a tubular heating zone, and crucibles, whose size is commensurate with the inner diameter of the tube, are continuously passed through the tubular portion in one direction for heating. In such a furnace, gas is not generated, and material can be subjected to heat treatment continuously.
However, in a method in which a furnace comprising a graphite tube serving as a heater is employed, for example, a crucible and powder contained in the crucible are heated by means of heat which is transferred or radiated from the tube. Therefore, in order to raise the temperature of material to be heated to approximately 3,000° C., the heater must be heated to a temperature greatly in excess of 3,000° C. However, at a temperature higher than 3,000° C., the heater itself is considerably consumed, and the service life of the heater is shortened. Incidentally, in order to treat a large amount of the material, the size of the crucible must be increased, and in accordance with an increase in crucible size, the size of the tube must be increased. In addition, the number of heaters must be increased, which causes an increase in equipment costs. Therefore, employing the method industrially is difficult.
There has conventionally been a method in which high-frequency is employed and heating is carried out by means of induction current. The method is efficient from the view that material contained in a crucible is continuously passed through a graphite tube. However, the material assumes the form of powder, and thus resistance of the material is too high for employment of induction heating of the material. Therefore, in order to heat the material, induction heating of the tube or the crucible must be
Murakami Shigeru
Sotowa Chiaki
Sudo Akinori
Hendrickson Stuart
Showa Denko Kabushiki Kaisha
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