Plastic and nonmetallic article shaping or treating: processes – Outside of mold sintering or vitrifying of shaped inorganic... – Of electrical article or electrical component
Reexamination Certificate
2000-10-12
2001-10-30
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Outside of mold sintering or vitrifying of shaped inorganic...
Of electrical article or electrical component
C264S618000, C264S620000, C264S642000, C264S645000
Reexamination Certificate
active
06309589
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method for manufacturing a pin heater that has a substantially internal insulating layer and an external conductive layer, the two layers comprising a ceramic composite structure.
BACKGROUND OF THE INVENTION
German Patent No. 35 19 437 and German Patent No. 37 34 274 have disclosed methods for manufacturing a pin heater from composite materials on the basis of trisilicon tetranitride (Si
3
N
4
) and molybdenum silicide (MoSi
2
) using an axial hot pressing technique, with separate sintered-in power supply wires. The complex construction and laborious manufacturing method of these pin heaters, in which green machining of the pin (before sintering) is not possible, but rather only hard machining with diamond tools after sintering is possible, are regarded as disadvantageous.
European Patent No. 0 721 925 describes a method for manufacturing a dense silicon nitride composite material. Although this material is temperature-resistant, electrically conductive composites cannot be created using the manufacturing method.
European Patent No. 0 601 727 describes a method for manufacturing a pin heater from Si
3
N
4
/MoSi
2
composites. It can be manufactured by hot pressing or sintering under 10
5
Pa of nitrogen at 1600° C. The composite material only begins to sinter at this temperature, resulting in relatively low strength values. This strength level is not sufficient for utilization of the pin heater, for example as a diesel starting aid. Complete sintering of the composite ceramic described is not possible under the manufacturing conditions indicated in European Patent No. 0 601 727. As a result, gas-tightness of the pins—which must exist if they are used, for example, as glow plugs—is also not ensured.
German Patent Application No. 197 22 321 describes a method for manufacturing a shaped element having adjustable electrical conductivity from a ceramic composite structure that contains at least two constituents of differing electrical conductivity, such as Si
3
N
4
and a metal silicide, the shaped element being produced before sintering by way of a cold isostatic mold pressing step. A method for manufacturing a pin heater is not, however, recited therein.
SUMMARY OF THE INVENTION
It is the object of the present invention to make available a method for manufacturing a pin heater that has a substantially internal insulating layer and an external conductive layer, that heats up quickly, that has excellent thermal and mechanical load-bearing capability at up to at least 1400° C., and that is gas-tight. In addition, heating rates of greater than or equal to 300 K/s to 900° C. are intended to be achievable, after application of a voltage of 10-15 V, with the pin heater manufactured according to the present invention. Above 1400° C., the pin heater is to exhibit modulation behavior. In addition, the power consumption of the pin heater manufactured in this fashion should not exceed 120 W in the initial phase.
According to the present invention, the object is achieved by a method for manufacturing a pin heater that has a substantially internal insulating layer and an external conductive layer, the two layers comprising a ceramic composite structure, wherein before the pin heater is sintered, its shaping is accomplished by way of the ceramic injection molding technique or by cold combined axial/isostatic pressing.
In an advantageous embodiment of the method, trisilicon tetranitride and a metal silicide are used as constituents of the ceramic composite structure.
Particularly advantageous in this context is a method in which 30-70 wt % Si
3
N
4
, 25-65 wt % MoSi
21
, 0-5 wt % Al
2
O
3
, and 2-9 wt % Y
2
O
3
are used as constituents of the ceramic composite structure.
The manufacture of a fast-heating pin heater with high strength includes the stage of shaping it and that of sintering it.
Shaping can be accomplished by way of the ceramic injection-molding technique, or by cold combined axial/isostatic pressing.
1. Shaping by CIM (Ceramic Injection Molding) In this technique, a preconditioned Si
3
N
4
powder equipped with corresponding sintering additives, such as Al
2
O
3
and Y
2
O
3
, is produced. This powder contains 0-5 wt %, advantageously 4.3 wt %, Al
2
O
3
and 2-9 wt %, advantageously 5.7 wt %, Y
2
O
3
. MSi
2
is mixed into this, wherein M can be molybdenum, niobium, tungsten, and titanium at various proportions by weight.
The admixture of MSi
2
is performed in such a way that after sintering, a highly insulating component and a very conductive component are created. The MoSi
2
concentration contained in the insulating component is in the range of 25-45 wt %, and in the conductive component is 50-65 wt %.
This is followed by the preparation of injection-moldable polymer compounds from the two components, i.e., ready-to-process mixtures of polymers with all the additives and fillers that are necessary for the manufacture of final products. These compounds are produced from the two preconditioned ceramic powder mixtures with a suitable organic binder system that, according to the present invention, has grafted polypropylenes in combination with cyclododecane and/or cyclododecanol. Also suitable as binder systems for producing the injection-moldable compounds are combinations of polyolefin waxes such as Hostamont® TPEK 583 of Ticona GmbH, or polyoxymethylenes such as Catamold® of BASF AG.
Injection-moldable powder compounds constitute highly filled dispersions. A binder system suitable for powder injection molding must meet the following requirements:
dispersive effect to prevent powder agglomerates;
good flow characteristics for the molten compound during injection molding;
sufficient adhesion (welding) when a second compound is overmolded onto a preform;
low level of pyrolysis carbon generation during thermal binder removal in an inert gas atmosphere or in air, since carbon has a deleterious effect on the properties of the sintered shaped element; and
rapid binder removal with no defect formation.
One such binder system is constituted by, for example, the combination of grafted polypropylenes with cyclododecane and/or cyclododecanol. The polar compounds grafted onto the polypropylene chain, such as acrylic acid or maleic acid anhydride, bind to the surfaces of the powder.
Lastly, the insulating element is injection-molded using the compound containing the insulating component. This insulating element is overmolded with the compound containing the conductive component by injection-bonding, resulting in welding of the two subelements. It is also possible to proceed in the opposite fashion.
It is particularly advantageous to injection-mold the insulating and conductive layers constituting the pin heater by two-component injection molding.
This is followed by an advantageously thermal binder removal process, and presintering under inert gas at 10
5
Pa at up to 1200° C.
2. Shaping by Cold Combined axial/isostatic Pressing
Shaping by cold combined axial/isostatic pressing can be performed using the two methods described below.
Method 2.1
First a preconditioned Si
3
N
4
powder is produced. This contains sintering additives such as Al
2
O
3
and Y
2
O
3
. MSi
2
, in which M can be molybdenum, niobium, tungsten, or titanium at various proportions by weight, is mixed into the powder. Organic pressing and/or binding auxiliaries, such as polyvinyl butyrals, polyvinyl alcohols, polyvinyl acetates, or polyethylene glycols, are also optionally added in an attrition mill in an organic solvent such as ethanol, propanol, or isopropanol. The preconditioned Si
3
N
4
powder contains 0-5 wt %, advantageously 4.3 wt %, Al
2
O
3
, and 2-9 wt %, advantageously 5.7 wt %, Y
2
O
3
.
The attrition-milled suspension is then dried in a rotary evaporator. Admixture of the MSi
2
is performed in such a way as to create one component that is highly insulating after sintering, and one component that is highly conductive after sintering. The former component contains, for example, MoSi
2
concentrations in the range of 25-45 wt %, and the latter component contain
Aichele Wilfried
Knoll Guenter
Lindemann Gert
Lindner Friederike
Schlachta Harry
Fiorilla Christopher A.
Kenyon & Kenyon
Robert & Bosch GmbH
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