Compositions: ceramic – Ceramic compositions – Refractory
Patent
1989-02-27
1991-07-09
Dixon, Jr., William R.
Compositions: ceramic
Ceramic compositions
Refractory
501153, C04B 3510
Patent
active
050306022
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to an alumina ceramic and, more particularly, to a ceramic made of alumina with a low alkali metal oxide content for heating conductors. Ceramic components with integrated heating conductors in the form of printed patterns consisting of a cermet are endangered by electrochemical decomposition of the carrier ceramic, of the ceramic cover layer and/or of the ceramic supporting structure. The electrolytic decomposition is produced by ionic conduction under the applied heating voltage, which is, for example, below 24 V direct-current voltage in the case of motor-vehicle applications, and can finally lead to the destruction of the heating conductor on account of the separation of cations with alloy formation and/or by separation of oxygen with oxide formation or by crack formation on account of the excess oxygen pressure arising below the insulating cover layer. This applies all the more the higher is the application temperature, and can become critical at temperatures as low as 600.degree. C. Such ceramic components with integrated heating conductors are used, for example, in sheathed-element glow plugs, in lambda probes or similar electrochemical sensors, in heated nozzles for various intended applications and in continuous-flow heaters for boilers or coffee machines in the form of small plate-shaped, pin-shaped or tubular heaters.
Aluminium oxide, as it is available in the trade, normally contains up to 0.5% by weight sodium oxide and/or potassium oxide. Usually, the aluminium oxide is used in this form, especially in mass-produced products, because the purer specifications of the aluminium oxide are far more expensive, as compared with those mentioned.
SUMMARY OF THE INVENTION
According to our invention an electrochemically stable ceramic used for making a ceramic support, a ceramic coating layer and a ceramic support skeleton for a heating conductor comprises alumina with alkali metal oxides and from 2.5 to 10 percent by weight of a flux consisting essentially of a barium silicate with a BaO:SiO.sub.2 molar ratio of from 2:1 and and 1:12.5, the sum of the Na.sub.2 O, K.sub.2 O, CaO and MgO content being less than 1 percent by weight, while the sum of the Na.sub.2 O and K.sub.2 O, CaO and MgO are initially present as impurities in the barium silicate and the alumina. Our invention has the advantage that it is electrochemically stable in long-term operation at the applied direct-current voltage at temperatures up to 1400.degree. C., that is to say that it shows no ceramic decomposition and/or heating-conductor damage due to cation migration or oxygen separation. Thus, of course, the appropriate components in which such a ceramic is used exhibit a substantially longer service life.
Advantageous further developments and improvements of the aluminium oxide ceramic indicated in the main claim are possible by means of the measures recited in the subclaims. It is particularly advantageous if the Al.sub.2 O.sub.3 ceramic contains less than 0.1% by weight Na.sub.2 O and K.sub.2 O respectively. Although both a ceramic without flux and also a ceramic with small quantities of flux may be produced from the aluminium oxide mentioned, a ceramic with a content of fluxes of 2.5-10% by weight is indeed particularly preferred, the proportions of Na.sub.2 O, K.sub.2 O, CaO amd MgO being below 1% by weight and ions which are relatively large and which therefore move only with difficulty, i.e. principally barium and strontium, being included in the flux, in other words the flux being present in the form of a barium silicate and/or of a strontium silicate. In this case, the BaO or SrO/SiO.sub.2 molar ratio can be between 2:1 and 1:12.5.
EXAMPLES
The text which follows contains a description of three embodiments, namely one for an Al.sub.2 O.sub.3 ceramic without flux, a second for an Al.sub.2 O.sub.3 ceramic with small quantities of flux and finally a third for an Al.sub.2 O.sub.3 ceramic with a flux consisting of barium silicate. Since the work-up of the mass took place in
REFERENCES:
patent: 2760875 (1956-09-01), Schwartzwalder et al.
patent: 2887394 (1959-05-01), Bickford et al.
patent: 3935017 (1976-01-01), Gardner
Frey Thomas
Friese Karl-Hermann
Geier Heinz
Bonner Melissa
Dixon Jr. William R.
Robert & Bosch GmbH
Striker Michael J.
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