SI23609A - One-stage process of manufacturing composite ceramic heater - Google Patents

Abstract

The subject of invention is a one-stage process of manufacturing composite ceramic heater before sintering powder mixtures of silicon nitride (Si3N4), Zirconium dioxode (Zn02) and oxide additives (Y203, Al203, Re203, ...) for sintering and organic additions to the design, while during sintering in an inert atmosphere of nitrogen or argon on the surface of the composite with the reaction of Si3N4 and ZrO2 forms ZrN, while core product remains unreacted. After sintering the product has the right mix insulating and conductive ceramic components, and good mechanical,thermal and electrical properties.

Description

ONE-STAGE PROCESS OF MANUFACTURING A COMPOSITE CERAMIC HEATER

The subject of the invention is a one-step process for manufacturing a composite ceramic heater prior to sintering a mixture of silicon nitride powder (S> 3N ₄ ), zirconium dioxide (ZrO ₂ ) and sintering oxide additives. During sintering at temperatures above 1600 ° C under a protective atmosphere of nitrogen or argon, a reaction between Si ₃ N ₄ and ZrO ₂ forms an electrically conductive zirconium nitride (ZrN) on the surface of the composite while leaving the core of the product unreacted. Therefore, after sintering, the product already has the correct combination of an insulating and conductive ceramic component with good mechanical and electrical properties.

View the problem

Because of their ability to operate at high temperatures, ceramic heaters are most commonly used in diesel engine ignition systems, as furnace lighters and as heating elements in heat machines and appliances. In recent times, ceramic glow plugs for diesel engines are by far the largest in terms of production, with the largest number of patents by well-known manufacturers such as NGK, Denso and Kyocera, Bosch, Beru, LeMark and others.

Regardless of their scope, ceramic heaters are generally made of at least two compatible materials, one of which is electrically conductive and the other electrically insulating. The conductive and insulating parts of the heater must match in the kinetics of thickening and the thermal expansion coefficient. They must have good thermal conductivity and the conductive part must have adequate electrical properties. The heater must be chemically, corrosion and mechanically resistant over a wide temperature range, and the prolonged operation and cyclic thermomechanical loads must not affect the change in the electrical properties of the ceramic.

-2Because of a favorable combination of chemical, thermal and mechanical properties, Si ₃ N ₄ based ceramics are the most suitable material for the non-conducting matrix component. The literature also mentions ceramic matrix based on Α osnovi-SiC, composites based on transition metal borides and SiAlON, but the mechanical properties and chemical stability of these matrix materials are significantly worse, so ceramic heaters of this composition are not (yet) commercially available. There are also some attempts to produce electrically conductive ceramics with an oxide matrix based on Al2O ₃ , S1O2 and Zr ₂ O ₃ , but such materials are not suitable due to poor thermal conductivity.

The electrically conductive component is usually obtained by dispersing an appropriate fraction of electrically conductive particles in an electrically nonconductive matrix with good chemical and mechanical properties. Conductive materials that may be considered suitable may be some refractory metals (eg W) and carbides, nitrides, carbonitrides, silicides, borides of the 3rd, 4th, 5th and 6th transition metal groups having high melting points. In practice, together with the Si ₃ N ₄ based matrix phase, they are most commonly used as the M0S12 and WC conductive phases. In addition to these two, we also find WSi2, NbSi ₂ , TiN, TiB ₂ , TiCN, TiC, ZrN, ZrB ₂ and TaN in the public and patent literature. It is important to provide a sufficient volume fraction of conductive particles in the conductive part of the ceramic heater to achieve a percolation threshold that depends on the particle size, size distribution and shape and is typically between 2535%. Because such a high proportion of dispersed particles of the conductive phase leads to poorer sintering and inadequate mechanical properties, intensive research has recently been conducted on materials with a smaller proportion of conductive phase, in an attempt to lower the percolation threshold, improve sintering and preserve the mechanical properties of the matrix phase as much as possible.

The state of the art

Patents in the field of ceramic heaters can be divided into two groups. The largest group of patents deals with various construction details, solving individual technical problems, such as. contact protection, more recently the ability to manage additional features. The second group contains patents protecting the composition and form

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-3 resistive part of the heater, and the patent claims indirectly also protect the method of forming a ceramic heater. For example, the composition and shape of type 2 NGK glow plugs are protected in EP 1477740, and the manufacturing process is described. The matrix phase is Si ₃ N ₄ based ceramics and conductivity is provided by the dispersed WC particles. The heater is designed to have more particles of conductive phase in the matrix in the lower part and a larger cross section, which provides higher conductivity. At the top of the heater, due to the smaller proportion of the conduction phase, the resistance increases and the heater also lights up. At the tip of the heater the volume content of the toilet is 16% and 20% in the rest. The conductive part of the heater is formed by two-component injection molding and casting into a porous model. The two-component injection molding tool consists of three parts, where after the first conductive phase is injected, the top of the mold is replaced and the next conductive component is injected. The pre-fabricated conductor portion of the heater is then placed in an insulating component that has been previously poured into a porous model. This is followed by cold isostatic compression, calcination, single-axis hot compression in graphite molds at 1900 ° C in an N ₂ atmosphere, mechanical and thermal treatment, and installation in a housing.

A slightly different design of the ceramic heater design was patented by Bosch (US 6710305). The patent states that suspensions are first prepared from the raw materials for each component of the heater after wet milling and mixing in polypropylene. Then, a high-pressure injection molds an insulating component which runs in the middle of the heater, and then a conductive component is sprayed, which wraps the insulating part of the heater. This is followed by cold isostatic compression of the entire heater, calcination and sintering with an elevated pressure of N ₂ at 1900 ° C. Patents US 6309581, US 6309589, US 6054680 address either a different combination of materials or individual geometric and structural enhancements to a ceramic heater, which in all cases is designed in several stages, e.g. by injection molding combined with cold compression.

The process of making a ceramic heater with a combination of multi-layer casting of aqueous suspensions is further known. The process was developed by Chinese-Canadian company ChongqingLeMark. The base material used for the matrix can be from the group (Si ₃ N ₄ , AIN, SiC, SiO ₂ , AI ₂ O ₃ , ZrO ₂ ), the conducting phase being represented by one of the known electrical conductors (MoSi ₂ , TiN, ZrN, TiCN, TiB ₂ ). US 5993722, US 6084212 and US 6884967

describe a method of manufacturing a multilayer ceramic heater with different mass concentrations of the conductive component in each layer, from 51-80% in the conductive core and from 0-28% in the outer insulation layer. The multi-layer casting of aqueous suspensions into the gypsum model is followed by cold pressing, drying and sintering in a protective atmosphere. The sintering conditions, i.e. temperature, pressure, atmosphere, are not mentioned in the patents, but sintering additives and deflocculants are used to prepare the suspension.

Regardless of the concept of the spark plug and the geometry of the heater, the common feature of all known patents is the process of two or more stages of injection molding and / or multilayer casting, which makes the design of the heater in many stages very time-consuming and technologically demanding, and during sintering or later during the operation of the heater, there may be problems due to incomplete contact between the insulating and the conducting components, leading to poor performance or even rupture.

The object and object of the invention is to provide a heater in such a way that, after sintering, it conducts electrical current on the surface, and the core of the heater is insulating, without the need for the heater to be shaped in several steps before sintering, as in the methods known to date.

According to the invention, the problem is solved by a one-step process of manufacturing a composite ceramic heater from a powder mixture containing Si ₃ N ₄ , ZrO ₂ and oxide sintering additives. During sintering in the presence of one of the known transient liquid phase, allowing densification Si ₃ N _4, with the reaction between the Si ₃ N ₄ and ZrO ₂ on the surface of the heating element forming ZrN as the conductive component, while the core of the heater remain unreacted, and therefore does not conduct electric flow. The ceramic heater according to the invention has good chemical, corrosion, oxidation and mechanical resistance, high strength and toughness and adequate electrical conductivity on the surface. In comparison with the known procedures, the production of the heater is thus simplified and inexpensive.

-5Description of the invention

Figure 1 shows the geometry and cross-section of an embodiment of a ceramic heater.

The process of manufacturing a ceramic heater according to the invention comprises the following steps:

Any basic silicon nitride powder with a large specific surface area and a small average particle size, in combination with one of the known oxide additive formulations (Υ2Ο3, AI ₂ O ₃ , Re ₂ C> 3, ...), is used as the basic building block to improve sintering, such as Y ₂ O ₃ and AI ₂ O3 by weight of from 1 to 15% and from 0.5 to 12%. It is desirable to keep these substances as pure as possible, since the presence of impurities adversely affects the properties of sintered ceramics. As a solid reagent for the formation of conductive ZrN grains on the surface of the heater, a nanometer or submicrometer ZrO ₂ powder with an average particle size of 6 nm to 40 pm, having a volume fraction of from 5 to 50% with respect to Si ₃ N ₄ , preferably from 10 to 10, is used 30%. ZrO ₂ nanometer particles can be formed by in situ precipitation of zirconium hydroxides on the surface of Si ₃ N ₄ particles from a solution of any zirconium salt in Si ₃ N ₄ suspension and subsequent calcination, and a commercially available ZrO ₂ nanoparticle suspension may be used. In the case of submicron particles, any commercially available ZrO ₂ powder containing no Y ₂ C> 3 in solid solution may be used. A powder mixture containing Si ₃ N ₄ , ZrO ₂ , one of the known combinations of oxide additives (Y ₂ C> 3, AI ₂ O ₃ , Re ₂ O ₃ , ...) for sintering and known organic additives required for dry or wet forming of blanks, it is dispersed in aqueous or non-aqueous wet media. After homogenisation, the suspension is formed into blanks in one step by any known method of dry or wet molding, such as pressing, casting and injection molding. After molding, the samples are dried or expelled from the organic additives required for molding and sintered at temperatures from 1600 ° C to 1900 ° C for 1 to 6 hours, preferably from 1750 ° C to 1850 ° C from 2 to 4 h at a flow of N ₂ or Ar, and subsequently machined. The inert atmosphere pressure can vary from 0.002 MPa to 20 MPa.

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-6 The invention will be further described below by way of example examples and Figure 1, showing the geometry and cross-section of a prototype ceramic heater, where the parts labeled 1 represent the conductive components and the part labeled 2 an insulating component.

Example 1

Separately prepare 200 ml of zirconium acetate (ZrAc) solution at a concentration of 0.5 mol per liter by diluting and mixing the commercial solution (manufactured by Swinton Manchester, United Kingdom) with distilled water and 500 ml of a suspension of 50 g Si ₃ N ₄ (manufactured by SN -E10, UBE Industries, Ltd., Japan) with an average particle size of 0.7 pm to 1.2 pm and 485 ml of distilled water by mixing in a planetary mill with Si ₃ N ₄ dishwasher bodies. The diluted ZrAc solution mixes well with the Si ₃ N ₄ suspension. After stirring for 20 minutes, the pH of the mixture was stabilized below 3.8 so that gas ammonia (manufactured by J. Medinger & Sohne, Austria) can be started to flow from 100 to 1500 ml / min. At a pH of 3.9, zirconium hydroxides already precipitate to the surface of silicon nitride powder, but their precipitating intensity is greatest at pH 7.0. The precipitate was filtered several times with distilled water and finally with absolute ethanol. The moist cake is air-dried at 80 ° C for 18 h and then thermally treated at 600 ° C for 2 h to form the crystalline ZrO ₂ form. This gives a Si ₃ N ₄ powder coated with ZrO ₂ nanoparticles with a volume fraction of 20% whose average size ranges from 8 nm to 10 nm. For blanket forming, Si ₃ N _{4 is} coated with ZrO ₂ nanoparticles together with the oxide additives Y ₂ O ₃ (HCStarck, Germany) and AI ₂ O ₃ (Alcoa, USA) with 5% and 3% by weight and organic polyvinyl alcohol (PVA) , Aldrich, Germany) for molding with a 2% by weight on a wet milled dry matter in a planetary mill in water for 3 h. After stirring, the resulting suspension is poured into a porous model and air-dried. The 5 mm x 42 mm x 3 mm squares are sintered in the backbone of a mixture of Si ₃ N ₄ and BN powder at 1850 ° C for 2 h at an argon flow of 0.1 MPa.

X-ray analysis of sintered specimens shows that ZrN, which is formed by the reaction between Si ₃ N ₄ and ZrO ₂ , is present in the outer section of the composite cross section (Fig. 1) while in the core • · · ·

7 unreacted ZrO _{2 present} . Quantitative Rietveld analysis of the XRD spectrum shows that 50% of all ZrO ₂ reacted in ZrN. The thickness of conductive component 1 on the sample surface is 0.5 mm ± 0.1 mm. The composite thus produced has a high density of 3.78 g / cm ³ and a flexural strength of 850 ± 50 MPa as measured by the four-point method. The distance between the supporting cylinders in this measurement was 10 and 20 mm (tensile side). The loading speed was 1 mm / min. A four-point DC method was used to measure electrical resistance. An In-Ga paste was used as the electrode for measuring electrical resistance. From the electrical resistance measured, electrical conductivity was calculated. The calculated electrical conductivity of the final composite was 8280 1 / Qm. In order to check the conditions under which the ceramic heater operates, the conductive layer on the lateral surfaces was first removed after sintering with diamond plates and thus designed any geometry of the prototype ceramic heater according to Figure 1. The test sample was connected to voltage is 12 V, with the tip of the sample glowing at a current of 8 A. The annealing temperature was measured with an OMEGA OS 3722 optical pyrometer (Omega Engineering, USA) and was 1300 ° C. The test showed that the material has a short response time, reaching a temperature of 1300 ° C in 2.2 seconds. The calculated resistance of the ceramic heater is 1.56 Ω, which corresponds to the required performance characteristics of the ceramic heater.

Example 2

Si ₃ N ₄ (SN-E10, Ube Industries, Japan), ZrO ₂ (TZ-0, Tosoh, Japan) with a volume content of 20% and oxide additives Y ₂ O ₃ were used as the starting materials for the composite ceramic heater. (HCStarck, Germany) and AI ₂ O ₃ (Alcoa, USA) with 5% and 3% by weight, respectively. 100 g of a powder mixture containing 58 g Si ₃ N ₄ and 34 g ZrO ₂ , 5 g Y ₂ O ₃ and 3 g AI ₂ O ₃ . It was ground wet for 5 h in an Si ₃ N ₄ bead mill (Φ = 3 mm) in isopropanol treated with 2 g of polyethylene glycol (PEG 400, Merck, Germany). After drying at 80 ° C, the powder was crushed and sieved through a plastic sieve to fine granulate. The powder mixture thus prepared was then compressed into test squares measuring 5 mm x 42 mm x 3

mm. Compressed blanks were further compressed isostatically with a pressure of 760 MPa and sintered in a dusting of S13N4 and BN at 1850 ° C2 h under a shielded nitrogen atmosphere at a nitrogen pressure of 0.1 MPa.

The results of X-ray and EDS analysis showed that, as in the first case, the reaction between S13N4 and ZrO ₂ also proceeds during sintering, with ZrN only forming on the surface of the product (Figure 1). A material with a volume content of ZrO _{2 of} 20% in the initial mixture exhibits a high density, 3.92 g / cm ³ and a flexural strength of 810 ± 30 MPa. The electrical conductivity of the sintered composite is 911 1 / Qm, which is suitable for the operation of a ceramic heater. By grinding the side surfaces, as in Example 1 (Figure 1), the conductive layer 1 is removed to form any geometry of the ceramic heater. After connecting the sample to a voltage of 12 V, the material ignites at the tip of the prototype with a current of 9 A, which gives a resistance of 1.33 Ω and an operating temperature of 1260 ° C. The product has a short response time as the operating temperature (1260 ° C) reaches a time of 2 seconds. The prototype produced shows that the product meets the essential requirements for the operation of the prototype ceramic heater.

The process of manufacturing a composite ceramic heater is thus characterized by the dispersion of a mixture of silicon nitride, zirconium oxide and one of the known combinations of oxide additives in a wet medium and, after homogenisation, the suspension is formed into blanks by one of the known dry or wet molding processes the molding, the samples are dried or expelled from the organic additives required for molding and sintered at temperatures from 1600 ° C to 1900 ° C for 1 to 6 h, preferably from 1750 ° C to 1850 ° C for 2 to 4 h in inert and / or reductive gas, preferably in N ₂ or Ar, wherein the atmospheric pressure may vary from 0.002 MPa to 20 MPa, or under vacuum. The mixture used contains silicon nitride and zirconium with a ZrO ₂ content of 5 to 50% by weight of Si ₃ N ₄ , preferably 10 to 30%, with an average particle size of 6 nm to 40 pm, plus one of the known combinations of oxide additives such as Y ₂ C> 3 and AI ₂ C> 3 by weight of from 1 to 15% and from 0.5 to 12% in the final mixture and known organic additives required for dry or wet forming of blanks .

Claims (3)

PATENT APPLICATIONS A one-step process for manufacturing a composite ceramic heater with an insulating (2) and conductive (1) ceramic component according to claim 1, characterized in that the mixture of silicon nitride, zirconium oxide and one of the known combinations of oxide additives is dispersed in a wet medium and that after homogenisation, the suspension is molded into blanks by one of the known methods of dry or wet molding; after molding, the samples are dried or expelled from the organic additives required for molding and sintered at temperatures from 1600 ° C to 1900 ° C from 1 to 6 h, preferably from 1750 ° C to 1850 ° C, from 2 to 4 h in inert and / or reductive gas, preferably in N ₂ or Ar, wherein the atmospheric pressure may vary from 0.002 MPa to 20 MPa, or in vacuum. Process according to claim 1, characterized in that the mixture contains silicon nitride and zirconium oxide with a ZrO ₂ content by volume of from 5 to 50% relative to Si ₃ N ₄ , preferably from 10 to 30%, with an average particle size of 6 nm up to 40 μιτι, and in addition one of the known combinations of oxide additives such as Y ₂ O ₃ and AI ₂ O ₃ with a weight ratio of 1 to 15% and from 0.5 to 12% in the final mixture and known organic additives, required for dry or wet blanking. A one-step process for manufacturing a composite ceramic heater with an insulating (2) and a conductive (1) ceramic component according to the preceding claims, characterized in that the conductive component is formed on the surface of the composite, while the core of the product remains unreacted and the product thus prepared with By grinding the side surfaces, it forms any geometry of the prototype ceramic heater, thereby achieving the proper performance characteristics of the ceramic heater.