DE102005035855A1 - Producing components, e.g. pistons for engines, comprises casting a mixture of mesophase carbon powder, liquid, gelling agent and additives followed by binder removal, carbonization and graphitization - Google Patents

Abstract

Producing components comprises casting a mixture of mesophase carbon powder, liquid, gelling agent and additives followed by binder removal, carbonization and graphitization.

Description

By its outstanding properties in terms of high-temperature stability, thermal shock resistance, corrosion resistance and its good tribological properties are graphite Wide field of application as a material in all industrial sectors. One Use in future key applications as in fuel cells or fusion reactors appears because above These properties are conceivable and probable [1, 2]. That Graphite as a structural material, such as a piston, still is not widely used, is mainly due to high production costs due. These Costs can through the use of a near net shape manufacturing process, such as it will be lowered below. Furthermore is it possible for the first time with these methods to design complex components Graphite with the possibility for undercuts without realizing extensive mechanical rework.

• • Zwischen 200–400 °C thermoplastische Eigenschaften, die ein Sintern des Materials ermöglichen. Somit kann das Porenvolumen deutlich reduziert werden.
• • Mit 90 % beste Kohlenstoffausbeute aller Ausgangsmaterialien, d.h. wenig flüchtige Bestandteile die zu einer Gasporosität im Werkstoff führen

Graphite has been produced by the Acheson process since 1895 [4]. High-carbon fillers such as petroleum or coal coke, carbon black or even natural graphite powders are mixed with tar or pitch and shaped by pressing. In a subsequent firing process at temperatures up to about 1000 ° C under an inert gas atmosphere, the binders are pyrolyzed and it remains pure carbon. The resulting gases diffuse out of the molding and leave a pore volume of up to 25% [4]. In order to achieve the highest possible final density, the carbonized bodies are infiltrated with liquid pitch as needed to reduce the pore volume and carbonized again. These end products, known as hard carbon, are graphitized at temperatures of 2500-3000 ° C in a time-consuming process and machined to the desired final shape. A disadvantage of these traditionally manufactured graphite bodies is the low mechanical strength, caused by a high porosity and an unfavorable microstructure. If graphite is to be used as a structural material with optimized mechanical properties, it is necessary to realize a largely pore-free carbon with a fine grain structure. Suitable starting materials are carbon mesophase powders [3]. These powders consist of spherulites with a diameter of several hundred nanometers, which in turn are composed of stacked, planar aromatics. They offer optimum conditions for graphitization, since the layer structure of the graphite is already pre-stamped. In addition, they are characterized by the following properties [5-11]:

• • Thermoplastic properties between 200-400 ° C, enabling sintering of the material. Thus, the pore volume can be significantly reduced.
• • With 90% best carbon yield of all starting materials, ie less volatile components which lead to a gas porosity in the material

The Application of modern injection molding techniques to highly filled MPK powder binder systems with subsequent Debinding and sintering steps could be the previous pressing process with all its disadvantages such as the high post-processing effort and the density inhomogeneities replace.

to Making a suitable feedstock is the selection of a matching binder system crucial. First restrictions result from the low softening points of the mesophases. Sintering operations of Mesophase powder in the component prior to pyrolysis of the binder would enter Escape of the decomposition gases during thermal debindering prevent and destroy of the sintered product through cracks.

aggravating is added that the mesophase only for high heating rates for sintering necessary thermoplastic behavior shows [6]. A suitable one Binder system must therefore as quickly as possible and without damaging the Remove structure from the sprayed component.

To date, there are no promising binder systems of other labor and research groups published, the these claims suffice.

patent

The use of a novel binder system for this material class, based on a liquid (polar content of up to 40%), an environmentally compatible gelling agent and auxiliaries (total of up to 20%) in small quantities, made it possible for the first time to successfully use a feedstock based on carbon mesophase powders are processed as organic graphite prematerial. Unlike conventional systems, most of the binder consists of liquid which can be removed by a simple drying step. Due to the drying shrinkage occurring (~ 13%, cf. 1 ) there is a first and decisive compaction of the component, which is absent in conventional, non-polar binder systems.

Alone through the homogeneous precompression taking place in this process step through the drying can the for achieved a successful sintering necessary powder densities also in injection molding become. About that Stay out with this first compaction with this system enough open pore channels around a gentle escape of the thermal decomposition products of the remaining binder components in the subsequent thermal debindering step.

By the low decomposition temperatures (<250 ° C) of the gelling agents and auxiliaries does not become the sensitive sintering step of the mesophase material disturbed and thus a higher one End density and far better mechanical properties than in reached all other previous binder systems.

advantages

in the Compared to alternative pressing methods are distinguished Injection molded components by a homogeneous density distribution and a low-stress and form-true sintering with homogeneous, defined Disappearance. Components of geometrically complex shape (e.g. Undercuts and threads) can only be done with this method getting produced.

Also it is impossible Components with the desired Properties such as high strength and fracture toughness and good thermal conductivity with conventional To produce binder systems.

Furthermore enable the low processing temperatures (<100 ° C) of this system make a simple, safe and energy-saving process control.

Of the successful use of powder injection molding in the metal and ceramic sector lets up a similar process stability and safety industrial processing of mesophase based systems shut down.

The first prototypes produced by injection molding on a commercially available plant show the following mechanical properties: Table 1 Mechanical properties of calcined / graphitized parts

Economic considerations

A transmission this process from the current state of development to industrial Scale could go down current knowledge within one year. decisive Development steps are the automation of compounding of the starting material and the drying of the green parts.

• • Lizenzierung des gesamten Prozesses an interessierte Unternehmen (Federal Mogul bzw. Schunk)
• • Gründung eines Spin-Offs mit der Spezialisierung auf Formteile in geringeren Stückzahlen (< 10 000/a).

With regard to the Central Institute for New Materials and Process Technology (ZMP), there are two possible strategies for recycling:

• • Licensing of the entire process to interested companies (Federal Mogul or Schunk)
• • Creation of a spin-off specializing in low-volume molded parts (<10 000 / a).

requirement

• • Kolben für den Einsatz in Verbrennungsmotoren,
• • Bipolarplatten zur Produktion von Hochleistungsbrennstoffzellen oder
• • Elektrodenmaterial in Li-Ionen-Akkumulatoren.

Specific considerations for using the described manufacturing process relate to the production of the following components:

• Piston for use in internal combustion engines,
• • bipolar plates for the production of high performance fuel cells or
• • Electrode material in Li-ion batteries.

Should in the long run, mass production of high-performance pistons succeeds to convert to graphite would alone at our cooperation partner Federal Mogul an annual Material volumes of 800 t / a arise. After the annual Study of the US Geological Survey (2002) [after 12] is the need on graphite for the use in fuel cells and batteries after a wide Market penetration even to 100 000 t / a rise.

7. Bibliography

• [1] Heuer, J., Development of Fine Grain Carbon Pistons in final report BMFT collaborative projects. 1,992th
• [2] Sato, S., et al., Neutron Irradiation effects on thermal shock resistance and fracture toughness of graphites as plasma-facing first wall components for fusion reactor devices. Carbon, 1989. 27 (4): p. 507-516.
• [3] Hoffmann, W.R., Über the sintering of polyaromatic mesophase for the production of high-strength Felnstkornkohlenstoffen. 1991, University of Karlsruhe.
• [4] Reynolds, W.N., Physical properties of graphite. 1968, Amsterdam, London, New York: Elsevier Publishing Co. Ltd.
• [5] Greinke, R.A. and L.S. Singer, Constitution of coexisting phases in mesophase pitch during heat treatment: Mechanism of mesophase formation. Carbon, 1988. 26 (5): p. 665-670.
• [6] Hoffmann, W.R. and K.J. Huttinger, Sintering of powders of polyaromatic mesophase to high-strength isotropic carbons-I. Influence of the raw material and sintering conditions on the properties of the carbon materials. Carbon, 1994. 32 (6): p. 1087 to 1103.
• [7] Mochida, I., Y. Sone, and Y. Koral, Preparation and properties of carbonaceous mesophase II highly soluble mesophase from ethylene tar modified using aluminum chloride as a catalyst Carbon, 1985. 23 (2): p. 175-178.
• [8] Nazem, F.F., Flow of molten mesophase pitch. Carbon, 1982. 20 (4): p. 345-354. [9] White, J.L., M.K. Gopalakrishnan, and B. Fathollahi, A processing window for injection of mesophase pitch into a fiber preform. carbon, 1994. 32 (2): p. 301-310.
• [10] White, J.L. and P.M. Sheaffer, pitch-based processing of carbon-carbon composites. Carbon, 1989. 27 (5): p. 697-707.
• [11] Rogovoi, V.N. and Y.B. Amerik, Raman scattering study of mesophase pitch. Vibrational Spectroscopy, 1993. 4 (2): p. 167-173.
• [12] Crossley, P., Graphite High-tech supply sharpens up: Industrial Minerals, no. 386, November 2000, p. 31-47.

Claims (10)

Process for the preparation of components of mesophase carbon powder ( 2 ), Liquid ( 3 ), Gel former ( 4 ) and excipients ( 5 ) by casting process ( 6 ) which subsequently gives birth ( 9 carbonized ( 10 ) and graphitized ( 10 ) can be. Method according to 1, characterized by the use of sinterable Mesophase smaller in a grain size 500 μm. Method according to 1, characterized by the use of organic or non-organic polar and non-polar liquids. Method according to 1, characterized by the use of gelling agents. Method according to 1, characterized by the use of excipients such as surfactants, dispersants, etc. Method according to 1, characterized by the use of casting processes in particular of injection molding. Process according to 1, characterized by a reduction or removal of the liquid binder components ( 3 ) of the shaped body by drying with or without the action of temperature and or humidity control of the surrounding atmosphere. Method according to 1, characterized by a density increase the molded parts due to the drying processes (7). Process according to 1, characterized by residual debinding of the component by thermal, catalytic, supercritical or solvent-technical Methods. Process according to 1, characterized by a carbonization up to 1200 ° C and a graphitizing treatment over these temperatures.