DE102008000100B4 - A process for producing a lightweight green body, then manufactured lightweight green body and method for producing a lightweight molded article - Google Patents

Abstract

Method for producing a lightweight green body in the form of a foam structure with three-dimensionally interconnected webs of a ceramic and / or powder metallurgical material, in which a suspension of at least one ceramic and / or powder metallurgical material, at least one binder and at least one suspending agent is produced, wherein a suspending agent is used, which sublimates below the melting temperature of the ceramic and / or powder metallurgical materials, subsequently either the suspension is subjected to a negative pressure until at least solidification of the suspending agent, or the suspension is subjected to a reduction in temperature until complete solidification of the suspending agent, then the solidified suspension is subjected to a reduction in pressure to a negative pressure, wherein during the pressure reduction, the temperature up to the melting point of the suspending agent in the jewei pressure is then increased, then the suspension is subjected to negative pressure until at least solidification of the suspending agent, and then the suspending agent is sublimated and completely removed from the green body and its surrounding space, during which sublimation the solidified structure of the suspension is obtained.

Description

The invention relates to the fields of powder metallurgy and ceramics and relates to a lightweight green body of a ceramic and / or powder metallurgical material, such as in vehicle construction, aircraft or mechanical engineering as a filter body, filler, lightweight component, catalyst support, pore burner, diesel particulate filter or depth filter for metallic melts, noise and vibration damper, compression element or lightweight component can be used, and a method for its preparation and a production of a lightweight molding.

Lightweight components are gaining in importance for a variety of designs in vehicle construction, aircraft construction or mechanical engineering for weight reduction. Due to their low density, foam structures are expected to be the lightweight components. The use of foam materials, optionally as composite components with dense outer parts, can be achieved by the weight reduction in vehicle construction, a considerable saving in fuel consumption.

Metal foams are characterized by a low mass in combination with a high rigidity and a significant mechanical strength. You will u. a. for thermal insulation, for noise and vibration damping or as compression elements, eg. B. used for "crash" absorber.

For the production of metal foams different methods are known, for. As the direct foaming of melts by gas injection or by the addition of foaming agents or foaming solid starting materials.

EP 1 288 320 A2 describes a melt metallurgical process in which gas bubbles are introduced into a melt. The size of the bubbles is controlled by the adjustment of the Einströmparameter of the gas.

An apparatus for introducing gas into a foamable metal melt by means of a metal foam producing tube in which the size of the respective individual bubbles and the size of the gas bubbles are controlled by geometrical nozzle design and adjustment of the gas inflow parameters into the molten metal in AT 411 532 B described.

AT 410 103 B Named a method for producing a lightweight molded body, wherein formed from a particle having molten metal by introducing gas or gas mixtures into this a metal foam, this at least partially introduced into a mold and its liquid phase is solidified in this.

To AT 408 078 B a method and an apparatus for producing bodies of metal foam are described by a powder metallurgy process in which a PM semi-finished in an at least two-part mold of metal, ceramic or the like inserted, and the closed mold is heated until the semifinished product melts in which the blowing agent incorporated in the PM semifinished product dissociates and gas is split off, so that the melt of the matrix metal foams and fills the mold, whereupon the mold is cooled and the melt penetrated by gas bubbles solidifies to a porous structure of low specific gravity.

Also, a powder metallurgical process for producing porous metal body is disclosed in DE 101 15 230 C2 in which a mixture comprising a powdered metallic material containing at least one metal and / or one metal alloy and a gas-releasing propellant-containing powder is compacted into a semi-finished product. This semifinished product is foamed under the action of temperature, wherein a propellant-containing powder is used, in which the temperature of the maximum decomposition is less than 120 K below the melting temperature of the metal or the solidus temperature of the metal alloy.

In US 4,713,277 B describes a process for the production of a metal foam with a specific gravity of 0.2 to 0.8 and evenly distributed polygonal cavities averaging 2 to 10 mm in size, in which initially the starting material for the molten metal (Al or alloys thereof) and Ca as Binder mixed and heated to melting temperature, then a frother, titanium hydride is mixed with 1 to 3 wt .-% of air.

Also known is after EP 1 338 661 B1 a method of producing a foamed / porous metal by adding a foaming agent to a molten pool of a metal forming a matrix, wherein a powder of a fluoride-coated carbonate compound is used as the foaming agent so that the fluoride covers an oxide film covering the matrix metal , and that carbon dioxide, which is formed by the carbonate compound and forms bubbles, can form outside metal oxide shells between the bubbles and the matrix.

A process for the production of metal foam from a foamable metal-containing raw material and a blowing agent which becomes effective on heating, in particular from one per se known compacted mixture of metal powder and propellant powder, with heating to at least the melting or foaming temperature of the raw material and blowing agent is in DE 197 34 394 C2 presented.

The use of gas-releasing blowing agents for the production of foams or metal foam composite components is also the subject of the patents EP 0 804 982 A2 . EP 1 392 875 B1 . US 5,865,237 A . DE 11 64 102 A . DE 197 44 300 A1 . DE 101 27 716 A1 . DE 199 27 837 C1 . DE 100 45 494 C2 and DE 101 04 339 A1 ,

In DE 101 31 041 C2 discloses a process for producing a foamed metal structure using a powder supply of metal particles which comprises introducing the stock of metallic powder particles together with foaming agent particles into a propellant gas to form a gas / particle mixture; spinning the mixture at a critical particle velocity onto a metallic substrate; and simultaneously or subsequently exposing at least the coating on the substrate to a thermal pass that causes expansion of the foaming agent to be activated while the metal particles are softened under the influence of the expanding plastic deformation gases.

From the EP 483 184 B For example, there has been known a method for producing a particle-reinforced metal foam, in which cell-forming gas is introduced into a melt of metal with finely divided reinforcing agents, foamed metal composite is formed, and from the surface of the molten material, the accumulated foam is removed and allowed to solidify.

In DE 10 2006 031 213 B3 discloses a process in which the stabilizing particles are produced in the preparation of the foamable starting material in an in-situ reaction of molten reactive liquids and a molten metal, wherein the molten metal added the constituents of the submicroscopic particles or nanoparticles to be produced at least as fluoride salt, then mixed and heated above the melting temperature of the mixture components.

In another method, the molten metal is mixed with solid, granular particles, which are dissolved out after solidification of the melt with solvents and thereby leave behind in the melt cavities. This prior art is z. As described in the following publications: WO 91/03 578 A1 . WO 92/03 582 A1 and WO 92/21475 A1 ,

All these methods of metal foam production have the common disadvantage that the foaming takes place at elevated temperature or even in the molten state of the metal, for which a large amount of energy is required. In addition, propellants are released, which may be explosive or harmful to the environment.

At room temperature, the following procedure works DE 10 2006 031 213 B3 , To produce steel foam from steel powder, water and a stabilizer, a slip is made at room temperature. Phosphoric acid is added to this mixture as a binding and blowing agent. Two reactions then take place in the slurry, leading to the formation of a stable foam structure. On the one hand, in the reaction between steel powder and acid, hydrogen gas bubbles are produced which cause foaming. On the other hand, a metal phosphate is formed, which solidifies the pore structure by its adhesive effect. The foam thus produced is dried and then sintered free of harmful substances to the metallic composite. A disadvantage of this method is that not all metallic or ceramic powder can be foamed with phosphoric acid, and phosphoric acid is not ecologically harmless. A possible uncontrolled release of hydrogen as a result of the reaction between acid and metal powder places further limits on this process.

A method deviating from the aforementioned methods is described in DE 102 38 384 B4 described. To produce the foam-like metal structure, use is made of a galvanizing method. This means that, before the electroplating step can be carried out, the surface of a non-conductive substrate with a foamed, ie pore-containing, structure must have been provided with a conductive surface.

In EP 0 558 142 B1 For example, a method for producing metallic foams is described in which a foam material is metallized via a treatment in an electrolytic nickel bath.

However, the described methods of metallization of foam structures disadvantageously always include the presentation of an already created foam structure of a particular material, which is subsequently coated with metal.

In the case of ceramic foams, it is above all the permeability of the pores and the thermal flow Properties in combination with the mechanical properties of utmost importance for the application. Such foam structures are used as pore burners, diesel soot filters, depth filters for metallic melts or catalyst carrier body.

Are known methods for the production of ceramic foams via infiltration of a polymer foam with a suspension consisting of dispersed ceramic particles in a liquid such. B. water DE 196 21 638 C2 , The polymer foam is burned out during sintering. The ceramic particles form an exact image of the polymer foam. Ceramic foams are also obtained via CVD processes by deposition on reticulated carbon foams.

• 1. das Bilden eines keramischen Schlickers, der ein im Wesentlichen homogenes Gemisch aus einem aus Partikeln bestehenden keramischen Stoff, einem organischen Bindemittel in einem flüssigen Trägermaterial und wahlweise einem oder mehreren grenzflächenaktiven Stoffen enthält, wobei mindestens einer der grenzflächenaktiven Stoffe vorhanden ist, falls das organische Bindemittel nicht als grenzflächenaktiver Stoff fungiert;
• 2. Aufschäumen des keramischen Schlickers mit Hilfe einer Kugelmühle; und
• 3. Erwärmen des aufgeschäumten keramischen Schlickers auf eine Temperatur, die ausreicht, um das organische Bindemittel im Wesentlichen auszubrennen.

In EP 1 117 626 B1 describes a process for the production of synthetic bone material, which consists of a large-pore foam ceramic whose open foam structure has pores with a modal diameter of ≥ 100 microns. The method comprises:

• 1. forming a ceramic slurry containing a substantially homogeneous mixture of a particulate ceramic, an organic binder in a liquid carrier material and optionally one or more surfactants, wherein at least one of the surfactants is present, if the organic Binder does not function as a surfactant;
• 2. foaming the ceramic slurry using a ball mill; and
• 3. heating the foamed ceramic slurry to a temperature sufficient to substantially burn out the organic binder.

Another method according to WO 03/008357 A2 works with an inorganic material, a metal powder or a metal paste, wherein in an aqueous ceramic mass a direct reaction with gas formation takes place between the minerals (eg kaolinites) and the metal powder (eg aluminum). The pore formation process takes place at room temperature or at higher temperatures in about 2 to 60 minutes. The resulting porous green compact is dried in the mold or using microwaves.

Another known method for producing an open-cell foam ceramic is the direct foaming by a blowing agent (product sheet foaming agent W 53 FL., Zschimmer & Schwarz GmbH & Co., Lahnstein). For this purpose, first a suspension of ceramic particles and water or a solvent is prepared. To this suspension, a blowing agent and polymer components are added. Subsequently, this suspension is poured into a mold and started the reaction of the blowing agent. This reaction leads to the development of gas bubbles, which lead to foaming of the suspension. Subsequently, the polymer components are crosslinked, whereby the foam solidifies. The polymer components are then burned out and the remaining foam is sintered. The disadvantage of this method is that the foaming is difficult to control.

Also known is a process for producing foam ceramic by direct foaming by means of air WO 97/45381 A1 , In this case, a polymer component is added to a suspension of ceramic particles and water or a solvent. Subsequently, air bubbles are introduced into the suspension by a high-speed special stirrer. The foamy suspension is then introduced into a mold, and by crosslinking the polymer component, the foam solidifies. Subsequently, the polymer component is burned out and the foam is sintered. By means of this method, only very fine foams can be produced, which have a low open-celledness. The strength of these foams is limited, since no high concentration of particles is possible, and when shrinkage during sintering also tensions and cracks occur, which limit the strength of the ceramic foam. In addition, the flowability of the foam is impaired because the pores are often closed cells.

Furthermore, from the DE 103 24 828 A1 Ceramic and / or powder metallurgical molded articles are known which consist of at least a partial volume of ice and a partial volume of frozen ceramic and / or powder metallurgical suspension and are prepared by a shaped body of ice coated with a suspension, dried the suspension and then carried out a sublimation drying or freeze drying becomes.

Also are from the DE 10 2007 000 053 A1 discloses a method and apparatus for producing a ceramic raw material comprising, in a slurry tank, a slip mixture composed of a ceramic particle dispersing liquid and a resin component dispersing liquid. The slip mixture is sprayed so that a frozen slip material is produced by sublimation. The frozen slurry material is dried by a vacuum dryer under a reduced pressure.

The object of the present invention is to specify a lightweight Green body made of a ceramic and / or powder metallurgical material, which has high mechanical strength and an adjustable cell structure and further in specifying a method for its production, with which the lightweight green or molded therefrom moldings are inexpensive and ecologically advantageous in a simple manner ,

The objects are achieved by the invention specified in the claims. Advantageous embodiments are the subject of the dependent claims.

In the inventive method for producing a lightweight green body in the form of a foam structure with three-dimensionally interconnected webs of a ceramic and / or powder metallurgical material, wherein a suspension of at least one ceramic and / or powder metallurgical material, at least one binder and at least one suspending agent wherein a suspending agent is used which sublimates below the melting temperature of the ceramic and / or powder metallurgical materials, subsequently either the suspension is subjected to a negative pressure until at least solidification of the suspending agent, or the suspension is subjected to a temperature reduction until complete solidification of the suspending agent , Thereafter, the solidified suspension is subjected to a reduction in pressure to a negative pressure, wherein during the pressure reduction, the temperature up to the melting point of the Sus pendissmittels is increased at the respective pressure, then the suspension is exposed to the negative pressure until at least solidification of the suspending agent, and then the suspending agent is sublimated and completely removed from the green body and its spatial environment, wherein during the sublimation, the solidified structure of the suspension is obtained ,

Advantageously, be used as a suspending agent, water or solvent.

Also advantageously be as suspending cyclohexane, cyclohexanol or tert. Butanol used.

It is also advantageous if the solidification is carried out by solidification or freezing of the suspending agent.

Further advantageously used as ceramic materials alumina, zirconia, mixed oxides, aluminum nitride, silicon nitride, silicon carbide, magnesium oxide, silica, porcelain, mullite, steatite, cordierite or other silicate materials.

And also advantageously used as powder metallurgy materials steels, titanium, aluminum or iron.

It is also advantageous if organic, temporary binders are used as binders.

And it is also advantageous if starch, polysaccharides, celluloses and cellulose derivatives, acrylates, polyvinyl butyral, polyvinyl alcohol, polyethylene glycol or sugars are used as organic, temporary binders.

It is also advantageous if glass powders, oxides or oxide mixtures, carbon-supplying organic compounds are used as known auxiliaries and additives.

It is also advantageous if a pressure of 4 to 600 Pa is applied as underpressure.

It is also advantageous if the suspension is introduced into a vacuum chamber or a freeze dryer.

And it is also advantageous if the suspension is simultaneously exposed to a negative pressure and temperatures below the solidification pressure and the solidification temperature of the suspending agent.

It is also advantageous if the solidified suspension is heated to the sublimation temperature of the suspending agent or slightly above it.

Further advantageously, the suspension is poured into a mold and exposed to the negative pressure in this form, and subsequently the suspending agent is sublimated from the solidified suspension in the mold and removed.

And it is also advantageous if, during suspension production, air and / or other gases are introduced into the suspension, more advantageously by means of stirring.

It is also advantageous if the negative pressure is set to solidification of the suspending agent within 1 s to 1 min.

It is also advantageous if the viscosity of the suspension controls the size and distribution of the cells between the webs, with a higher-viscosity suspension resulting in a smaller size and distribution of the cells between the webs.

• – einem keramischen und/oder pulvermetallurgischen Material sowie aus Bindemittelpartikeln
• – oder aus einem keramischen und/oder pulvermetallurgischen Material sowie Bindemittelpartikeln und Hilfs- und Zusatzstoffen für die Sinterung von keramischen und/oder pulvermetallurgischen Materialien,

wobei keinerlei Materialien zur Erzeugung der Zellen zwischen den Stegen der Schaumstruktur vorhanden sind, und wobei die Stege die Erstarrungsstrukturen eines sublimierten Suspendiermittels aufweisen. The lightweight green body according to the invention in the form of a foam structure with three-dimensionally interconnected webs consists of

• - A ceramic and / or powder metallurgical material and binder particles
• Or of a ceramic and / or powder metallurgical material as well as binder particles and auxiliaries and additives for the sintering of ceramic and / or powder metallurgical materials,

wherein there are no materials for generating the cells between the ridges of the foam structure, and wherein the ridges have the solidification structures of a sublimed suspending agent.

Also advantageously present as ceramic materials are alumina, zirconia, mixed oxides, aluminum nitride, silicon nitride, silicon carbide, magnesia, silica, porcelain, mullite, steatite, cordierite or other silicate materials.

Further advantageously, as powder metallurgical materials steels, titanium, aluminum or iron are present.

Advantageously, the webs in addition to ceramic particles and / or metal particles and binder particles from auxiliaries and additives for the sintering of ceramic and / or powder metallurgy materials.

Also advantageously as binders organic, temporary binders are present, which are still advantageously starch, polysaccharides, celluloses and cellulose derivatives, acrylates, polyvinyl butyral, polyvinyl alcohol, polyethylene glycol or sugars.

And likewise advantageously, glass powders, oxides or oxide mixtures, carbon-supplying organic compounds are present as known auxiliaries and additives.

In the method according to the invention for producing a lightweight molding of a ceramic and / or powder metallurgical material, in which a green body is sintered at temperatures below the melting temperature of the ceramic and / or powder metallurgical materials.

The lightweight green and shaped bodies according to the invention have a structure of open and closed cells, which are formed by the webs. The webs are made of a ceramic and / or powder metallurgical material, wherein the green body still binder and / or auxiliaries and additives for the sintering of the materials may be present.

The lightweight green bodies of the invention consist of a foamed or porous body having a specific mass of 10 to 90% of the pure material with a plurality of closed, isolated or open, partially interconnected, generally spherical cells or a combination of both cell types with cell sizes Range of 10 microns to 10 mm, all of these cells are formed by the webs. These cells are not the pores that are in the web material.

Compared with metallic foams which have been produced by melt processes, the lightweight moldings according to the invention differ in that they obtain their final strength via sintering. While the metallic foams have a solidification structure, the structures according to the invention of the shaped bodies show a material-typical sintered structure.

Compared with the metallic foams prepared with phosphoric acid as a propellant, the structures according to the invention of the green and shaped bodies have no phosphate binder phases in the microstructure.

Compared with ceramic and metallic foam structures which are produced by coating polymeric foam skeleton structures with powder suspensions, the structures according to the invention of the green and shaped bodies are characterized by the fact that they have no hollow webs, which represent replicas of the burned-off polymer structures.

The structures of the green bodies according to the invention can be distinguished from all other ceramic and metallic foam structures in that the webs of the green bodies have surface features and pores resulting from the solidification of the suspending agent and the often associated crystal growth. In the case of water as a suspending agent, the webs of the green body have the structures of the sublimated ice crystals. These characteristic structures of erosion can, for example, be detected in the scanning electron microscope and give an indication of the genesis of the particular foamed, porous structure (Figs. 1-5).

Figure 1: Bar of an unsintered alumina foam structure with pores of ice crystals

Figure 2: Bar of an unsintered alumina foam structure with pores of ice crystals

Figure 3: Bar of an unsintered alumina foam structure with pores of ice crystals

Figure 4: Bar of a unsintered steel foam structure with pores of ice crystals

Figure 5: Bar of a unsintered steel foam structure with pores of ice crystals

From ceramic and / or powder metallurgical materials, a suspension with solids contents of> 1 to 98% by volume is produced with a suspending agent and binders and optionally auxiliaries or additives for the sintering of the materials. Such a suspension used according to the invention may have a viscosity of slightly liquid to pasty due to the solids contents.

The suspension can be stirred advantageously. The amount of introduced air or other gases in the suspension and the viscosity of the suspension can control the number and size of cells formed by the webs.

The suspension is filled into molds, hollow component structures or vessels and transferred to a device in which the suspension can be exposed to a negative pressure. This is advantageously a vacuum vessel or a freeze dryer.

By the method according to the invention, the suspension produced, while it is subjected to a negative pressure until at least solidification / solidification / freezing of the suspending agent, foamed. By applying the negative pressure, the air or other gases inevitably present in the suspension is removed / sucked out of the suspension. As a result of this and due to the lowered boiling point of the suspending agent due to the reduced ambient pressure, the suspension foams up and results in a lightweight highly porous structure up to a foam structure. The foaming of the suspension can also be achieved by advantageously solidifying / solidifying / freezing the suspension in a mold, a hollow component structure or a vessel. This solidified / solidified / frozen suspension is then subjected to a reduced pressure, whereby the solidified / solidified / frozen suspension is heated above the melting temperature of the suspending agent. By at least partial liquefaction of the suspending agent, the suspension can foam again. As a result of the further pressure reduction, a balance between pressure and temperature is again achieved, so that the foamed suspension is then solidified / solidified / frozen again.

Subsequent to the preparation of the foam structure, the sublimation of the suspending agent is carried out and the green body is produced.

Furthermore, according to the invention, the suspension of the ceramic and / or metallic powders with a gel-forming substance such. As colloidal or polymeric sols, agar or gelatin are added. After applying a negative pressure to the suspension during the evacuation of the vessel, the formation of bubbles in the suspension, which leads to the inventive lightweight porous to foamed structure. When gels are used as suspending agents, advantageously the webs of the green body have the structures of the gel structure. By further lowering the ambient pressure, for example to pressures <600 Pa, the equilibrium conditions are achieved, which causes solidification / solidification / freezing of the structure. At the same time, the gel formation causes an additional solidification of the structure.

The solidified / solidified / frozen foam is then subjected to sublimation conditions and the suspending agent is removed to form the green body.

In the special case of the use of gel-forming substances, however, the foam structure according to the invention can also be removed from the vacuum vessel after solidification / solidification / freezing under ambient pressure and warmed to room temperature. Although the foamed or porous structure is not yet dried at this time, and the pore space between the lands is filled with the suspending agent, preferably water or solvent, the resulting gel imparts solid properties to the molding. The residual moisture can now be removed from the structure by conventional drying methods, such as evaporation or evaporation drying, whereby the green body according to the invention is formed.

The green body according to the invention is subsequently subjected to a sintering process which leads to the solidification of the ceramic or metallic powder particles and gives the foam structure its final solid-state properties.

Advantageously, the suspension can be immediately filled in a mold and these are evacuated, so that the resulting green body is also a molded green body.

Also advantageously, an aqueous based suspending agent is used.

But it is also possible, instead of water as suspending agent for the suspension of organic solvents, for. B. tertiary butyl alcohol, Use cyclohexane or cyclohexanol. According to the state diagrams of these solvents compared to that of the water, the properties of vapor pressure, melting point and boiling point then change, which must be taken into account during the process. The solvents used must meet the condition that the pressure in a known device can be lowered below the equilibrium pressure of the melting temperature of the solvent. Only then can the phase transition to the solid be completed. If there is no such device, the solvent can not be used as the suspending agent.

Further advantageously, the evacuation is carried out in a vacuum vessel or a freeze dryer.

Due to the further lowering of the pressure until solidification / solidification / freezing of the suspending agent, this solidifies / freezes / freezes and the lightweight, highly porous structure until foam structure of the suspension is maintained. The solidification / solidification / freezing of the suspending agent is achieved by the achievement of an equilibrium pressure which corresponds to the temperature at the freezing point of the suspending agent. The solidification / solidification / freezing of the suspending agent is, as it were, the solidification or the freezing of the suspending agent.

The solidified or solidified or frozen foamed suspension as a green body can now also be processed by means of physical or chemical processes, advantageously by means of mechanical processes, whereby care must be taken during this processing that the suspending agent does not melt, ie loses the solidified state.

Subsequently, the solidified / solidified / frozen suspending agent of the green bodies according to the invention is sublimated. In this case, the pressure and temperature are adjusted so that in any case no liquefaction of the suspending agent can take place and, at the same time, the suspending agent is sublimated as completely as possible.

The sublimed suspending agent is removed from the body and from its spatial environment. This can advantageously be done in a freeze dryer, from which the gaseous suspending agent is removed by suction.

Thereafter, the green body according to the invention can be exposed to the ambient conditions.

The sublimation, as well as the foaming and the solidification / solidification / freezing of the suspending agent, can be controlled by pressure and temperature.

The equilibrium conditions for the foaming, the solidification / solidification / freezing of the suspending agent and the sublimation are known for the suspending agents which can be used according to the invention.

In the case of using water as a suspending agent, the pressure at room temperature is lowered to ≦ 600 Pa, which corresponds to the triple point of the water. As a result, foaming of the aqueous suspension is achieved and, at the same time, the water freezes upon reaching this pressure. The green body with the frozen foam structure is then exposed to a pressure of 4 to 250 Pa. Within this pressure range, the water sublimes at a shelf temperature of 20-25 ° C. The sublimation rate can be increased when the temperature is raised to 40-60 ° C. This can be done for example via an additional heat source, eg. B. be supported by a heat radiator, by induction heating, by microwave energy or by a hot plate.

To increase the strength of the foam structure dried as a result of the sublimation, customary temporary binders can be added to the suspension. The green body according to the invention in the form of a dried foam, which does not undergo any drying shrinkage during the sublimation drying, is subjected to a sintering process at the connection, which leads to the solidification of the ceramic or metallic powder particles and gives the foam structure its final solid-state properties. It should be noted that the sintering conditions does not allow melting of any of the components, otherwise the foamed structure would be lost.

The suspensions may advantageously be added freezing point-lowering substances, such as. As glycerol, ethanol, acetone, isopropanol. In this way, a foaming of the suspensions can be achieved even at lower ambient pressures than <600 Pa.

The lightweight molded body produced according to the invention from an at least highly porous ceramic and / or powder metallurgical material consists of an at least highly porous structure of three-dimensionally interconnected webs which consist entirely of a sintered ceramic and / or powder metallurgical material and have substantially no pores and / or cavities, wherein there are no cracks, voids and pores of removed materials to produce the at least high porosity.

In this case, the three-dimensionally interconnected webs of alumina, zirconia, mixed oxides, aluminum nitride, silicon nitride, silicon carbide, magnesium oxide, silica, porcelain, mullite, steatite, cordierite or other silicate materials, advantageously of steels, titanium, aluminum or iron.

Further advantageously, the density of the three-dimensional interconnected sintered web material ≥ 80% of the theoretical density.

As an advantage over the known methods of metal foam production, the green and shaped bodies according to the invention are produced in the highest degree environmentally friendly, since no propellants must be used for gas formation. Furthermore, the method described works very energy-saving, since the production of highly porous to foam structure is not carried out in the melt or by thermal decomposition of the blowing agent, but at temperatures around the freezing point of the suspending agent.

As a further advantage over the known methods, it should be mentioned that the size and distribution of the cells formed by the webs can be adjusted precisely by regulating the pressure and temperature of the suspension.

The cell volume can be controlled via the preparation of the suspension, the viscosity of the suspension, the solids content of the suspensions and likewise via the process control of pressure and temperature. The formation of the cells can be abruptly interrupted by the pressure control by solidification / solidification / freezing of the suspension.

As advantages over the known methods of foam ceramic manufacture, it should be mentioned that the foam structures do not require burning out of a polymer for the cell structure or an organic spacer, as cell formers advantageously steam and ceramic full webs can be achieved. A thermal treatment of the material for foaming due to blowing agent decomposition is not required.

Furthermore, it is advantageous that the shaped bodies are sintered at temperatures below the melting temperatures of the components. This results in no melting phase, whereby the highly porous to foam structure is maintained and on the other hand, no conversion processes of the materials can take place.

The invention will be explained in more detail below with reference to several exemplary embodiments.

Example 1:

428 g of alumina powder (A-16, ALCOA) with a particle size of d ₅₀ = 0.4 microns are dispersed in 75 g of water with the addition of 5 ml of a condenser (Dolapix CE64 Zschimmer & Schwarz) with stirring in an ultrasonic bath. As a temporary binder, 10 ml of 15% Moviol solution are added to the suspension. After stirring for 1 hour at room temperature, 10 ml of the suspension having a solids content of 85% by mass are introduced into a cylindrical silicone rubber mold (∅ 25 mm) at room temperature and are positioned on the support surface of a freeze dryer (Gamma 20, Martin Christ). Subsequently, the pressure in the vacuum chamber of the freeze dryer is lowered to a pressure of 125 Pa, which corresponds to an equilibrium temperature of the ice of -18 ° C. During the pressure reduction, the suspension is foamed. Upon reaching the freezing point of the suspension of the resulting foam freezes abruptly. After reaching the vacuum pressure of 125 Pa, the surface heating is set to 30 ° C and freeze-drying is started. After 48 hours, the lyophilization process is stopped and the dried foam sample removed as a green body. The temporary binder is removed at 500 ° C. Subsequently, the sintering is carried out in air at a heating rate of 3 K / min at 1700 ° C for 1 h hold time. The foamed structure thus obtained has a density of 56% of the density of alumina (3.99 g / cm ³ ). The proportion of open porosity between the webs is 35%.

Example 2:

415 g of aluminum oxide powder (A-16, ALCOA) with a particle size of d ₅₀ = 0.4 μm are dispersed in 80 g of water with the addition of 5 ml of a disposable cleaner (Dolapix CE64 (Zschimmer & Schwarz) with stirring in an ultrasonic bath.) As a temporary binder 10 ml of 15% Moviol solution are added to the suspension, and 10 ml of 83.86% by weight of the suspension are poured into a cylindrical silicone rubber mold (∅ 25 mm) at room temperature for 1 h and placed on a footwell Then the pressure in the vacuum chamber of the freeze dryer is lowered to a pressure of 125 Pa, which corresponds to an equilibrium temperature of the ice of -18 ° C. During the pressure reduction, the suspension is foamed After reaching the freezing point of the suspension, the resulting foam freezes abruptly After reaching the vacuum pressure, the surface heating is set to 30 ° C lt and the freeze-drying started. After 48 hours, the lyophilization process is stopped and the dried foam sample removed. The temporary binder is removed at 500 ° C. Subsequently, the sintering is carried out in air at a heating rate of 3 K / min at 1700 ° C for 1 h hold time.

The resulting foamed structure has a density of 45% of the density of alumina (3.99 g / cm ³ ). The proportion of open porosity between the lands is 48%.

Example 3:

460 g of steel powder (Micro Melt 17-4 PH, Carpenter) with a particle size of d ₉₀ <22 microns are dispersed in 33.5 g of water by stirring in an ultrasonic bath. 15 ml of a dispersing aid (Dolapix CE64, Zschimmer & Schwarz) used. In addition, 2.5 ml of glycerol are added to the suspension to lower the freezing point. The solids content of the suspension is 92.9% by mass. The suspension is then filled into silicone rubber molds and frozen by contact with a metal plate (-196 ° C) cooled in liquid nitrogen. Until the transfer to the freeze dryer, the frozen suspensions are stored for 24 h in a freezer at - 21 ° C. The frozen cylindrical samples (∅ 25 mm, h = 6 mm) are then demolded and positioned in metal dishes on the freeze-dryer shelves at room temperature. The pressure in the vacuum chamber of the freeze dryer is lowered immediately thereafter to 125 Pa. During the pressure reduction, the frozen samples are heated by contact with the warmer shelves to the melting point of the water, causing the suspension to foam. After a decrease in the pressure in the vacuum chamber to the equilibrium pressure of the water, which corresponds to a temperature below the melting point of the suspension, there is an immediate freezing of the water and thus the resulting foam structures. After reaching the pre-set vacuum pressure, the shelf heating is set to 30 ° C and freeze-drying is started. After 48 hours, the lyophilization process is stopped and the dried foam sample removed. The removal of the temporary binder was carried out at 700 ° C under Ar atmosphere with 5% H ₂ . The sintering is carried out under hydrogen at a heating rate of 3 K / min at 1380 ° C for 1 h hold time. The density of the sintered foam structure is 2.87 g / cm ³ , which corresponds to 36.8% of the theoretical density of steel grade 17-4 at 7.8 g / cm ³ . The proportion of open porosity between the webs is 60%.

Claims (26)

Method for producing a lightweight green body in the form of a foam structure with three-dimensionally interconnected webs of a ceramic and / or powder metallurgical material, in which a suspension of at least one ceramic and / or powder metallurgical material, at least one binder and at least one suspending agent is produced, wherein a suspending agent is used, which sublimates below the melting temperature of the ceramic and / or powder metallurgical materials, subsequently either the suspension is subjected to a negative pressure until at least solidification of the suspending agent, or the suspension is subjected to a reduction in temperature until complete solidification of the suspending agent, then the solidified suspension is subjected to a reduction in pressure to a negative pressure, wherein during the pressure reduction, the temperature up to the melting point of the suspending agent in the jewei pressure is then increased, then the suspension is subjected to negative pressure until at least solidification of the suspending agent, and then the suspending agent is sublimated and completely removed from the green body and its surrounding space, during which sublimation the solidified structure of the suspension is obtained. Process according to Claim 1, in which the suspending agent, water or solvent is used. A method according to claim 2, wherein the suspending agent is cyclohexane, cyclohexanol or tert. Butanol be used. Process according to claim 1, wherein the solidification is carried out by solidification or freezing of the suspending agent. The method of claim 1, wherein as the ceramic materials alumina, zirconia, mixed oxides, aluminum nitride, silicon nitride, silicon carbide, magnesium oxide, silica, porcelain, mullite, steatite, cordierite or other silicate materials are used. Process according to claim 1, in which steels, titanium, aluminum or iron are used as powder metallurgical materials. Process according to Claim 1, in which organic, temporary binders are used as binders. Process according to claim 7, in which, as organic, temporary binders starch, Polysaccharides, celluloses and cellulose derivatives, acrylates, polyvinyl butyral, polyvinyl alcohol, polyethylene glycol or sugars are used. Process according to Claim 1, in which auxiliaries and additives which are advantageously glass powders, oxides or oxide mixtures, carbon-yielding organic compounds are used. A method according to claim 1, wherein a pressure of 4 to 600 Pa is applied as negative pressure. The method of claim 1, wherein the suspension is introduced into a vacuum chamber or a freeze dryer. A process according to claim 1, wherein the suspension is simultaneously exposed to a negative pressure and temperatures below the solidification pressure and the solidification temperature of the suspending agent. The process of claim 1 wherein the solidified suspension is heated to the sublimation temperature of the suspending agent or slightly above. A process according to claim 1, wherein the suspension is poured into a mold and subjected to negative pressure in this form, followed by sublimation and removal of the suspending agent from the solidified suspension in the mold. A method according to claim 1, wherein air and / or other gases are introduced into the suspension during suspension production. Process according to claim 15, wherein the suspension is stirred during production. The method of claim 1, wherein the negative pressure is set to solidify the suspending agent within 1 second to 1 minute. The method of claim 1, wherein the viscosity of the suspension controls the size and distribution of the cells between the ridges, with a higher viscosity suspension resulting in a reduced size and distribution of cells between the ridges. Lightweight green body in the form of a foam structure with three-dimensionally interconnected webs, consisting of - A ceramic and / or powder metallurgical material and binder particles Or of a ceramic and / or powder metallurgical material as well as binder particles and auxiliaries and additives for the sintering of ceramic and / or powder metallurgical materials, prepared according to at least one of claims 1 to 18, wherein no materials for producing the cells are present between the webs of the foam structure, and wherein the webs have the solidification structures of a sublimated suspending agent. A green body according to claim 19, wherein there are present as ceramic materials alumina, zirconia, mixed oxides, aluminum nitride, silicon nitride, silicon carbide, magnesia, silica, porcelain, mullite, steatite, cordierite or other siliceous materials. Green body according to claim 19, in which there are present as powder metallurgy materials steels, titanium, aluminum or iron. Green body according to claim 19, in which the webs in addition to ceramic particles and / or metal particles and binder particles from auxiliaries and additives for the sintering of ceramic and / or powder metallurgical materials. Green body according to claim 19, in which organic, temporary binders are present as binders. Green body according to claim 23, in which starch, polysaccharides, celluloses and cellulose derivatives, acrylates, polyvinyl butyral, polyvinyl alcohol, polyethylene glycol or sugars are present as organic, temporary binders. Green body according to claim 19, in which glass powders, oxides or oxide mixtures, carbon-supplying organic compounds are present as known auxiliaries and additives. A process for producing a lightweight molding of a ceramic and / or powder metallurgical material, wherein a green body is sintered according to at least one of claims 19 to 25 at temperatures below the melting temperature of the ceramic and / or powder metallurgical materials.

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