Electric heating – Heating devices – With heating unit structure
Reexamination Certificate
2002-10-03
2004-08-03
Look, Edward K. (Department: 3745)
Electric heating
Heating devices
With heating unit structure
C219S553000
Reexamination Certificate
active
06770856
ABSTRACT:
The invention relates to the provision of materials for use in electric heaters, parts, sensors and tools operating in oxidative media at 1000-1900° C. The proposed heat-resistant material is suitable for manufacturing individual parts, high-temperature protective coatings and high-temperature soldered joints of part components which, in their turn may be manufactured from other high-temperature materials: refractory materials and alloys based thereon, carbon and silicon-carbide materials, as well as composite materials based on suicides of refractory metals “REFSIC”. The proposed heat-resistant material may be used for producing composite materials and articles therefrom with the use of other high-temperature materials in various combinations.
Known in the art are silicon carbide electric heaters with a known protective coating material, described in [SU 1694552 A1, C04B 35/56]. The protective coating is produced by applying a suspension based on molybdenum disilicide, followed by roasting. Introduced into the suspension are 75-85% of molybdenum disilicide and 15-25% of zirconium oxide stabilized with yttrium oxide, the ratio of these oxides being 9:1. The same ratio of the components is preserved almost unmodified in the material of finished protective coating which may have a thickness of up to 200-250 &mgr;m. The coating of a greater thickness peels off and degrades in the course of temperature cycling; if the thickness of the coating is smaller, the service life of the coating material and of the whole heater under oxidative conditions at high temperatures is noticeably reduced.
A disadvantage of such material is its low stability. The thickness of the coating cannot be increased without formation of cracks because of a considerable difference in the values of thermal expansion coefficients of silicon carbide (&agr;=(4-4.6)×10
−6 1
/deg [V. V. Vikulin, Structural and Functional Ceramics, Obninsk, 1997, Institute of Nuclear Power (in Russian)]), which constitutes the basis of the heater, and of tetragonal molybdenum disilicide (&agr;
a
==8.2×10
−6 1
/deg, &agr;
c
=9.4×10
−6 1
/deg), which, in its turn, constitutes the basis of the coating material. The oxide phase in the coating has a still higher thermal expansion coefficient than molybdenum disilicide has. As a result, the coating cracks easily under the effect of temperature cycling at a rate higher than 20° C./second, and the heater fails.
Known in the art are silicon carbide electric heaters comprising a known heat-resistant protective coating material produced by powder metallurgy techniques [SU 1685752, H05B 3/14]. The coating material comprises a sublayer of molybdenum silicides Mo
3
Si and Mo
5
Si
3
having a thickness of 180-220 &mgr;m and an outer sublayer of molybdenum disilicide (MoSi
2
) having a thickness of 150-250 &mgr;m. The total thickness of the protective coating layers cannot be increased to exceed about 500 &mgr;m because of crack formation. To increase the service life at 1500-1600° C. and under temperature cycling conditions in an oxidative medium, the coating comprises two layers: a sublayer from lower molybdenum suicides Mo
3
Si and Mo
5
Si
2
contains them in the ratio of 1:5, and a layer based on molybdenum disilicide further comprises 20-30% of an oxide filler from a mixture of zirconium and yttrium oxides in the ratio of 95:5 and sodium aluminate with the following ratio of the components in the oxide filler: mixture of zirconium and yttrium oxides, 50-90%; sodium aluminate, 10-50% by weight.
The main disadvantage of the material in the form of a two-layer coating is its low stability upon temperature cycling at the rate of heating and cooling higher than 20° C./second and also at temperatures of 1600-1700° C. and higher. The thickness of the coating, limiting its service life, increased over SU 1694552 A1 to 470 &mgr;m, cannot be further increased markedly without formation of cracks because of considerable difficulties in the thermal expansion coefficients of the silicon carbide, the sublayer from lower molybdenum silicides and the layer of molybdenum dicilicide. This circumstance limits the stability of the coating material and of the whole electric heater under temperature cycling conditions, especially at high rates thereof.
It is known to use molybdenum dicilicide [GB 2015910 A] as a cement for joining carbon articles.
The main disadvantage of molybdenum disilicide used for cementing together carbon articles is low stability of the cemented joint. Under temperature cycling conditions, cracks are easily formed on the thus cemented articles because of a large difference in the thermal expansion coefficient between molybdenum disilicide and carbon materials.
It is known to use an eutectic of molybdenum suicides MoSi
2
+Mo
5
Si
3
as a high-melting solder for soldering refractory metals [G. B. Cherniack, A. G. Elliot, High-temperature behavior of MoSi
2
and Mo
5
Si
3
, Journal of the American Ceramic Society, vol. 47, No. 3, pp. 136-141.a].
The main disadvantage of the eutectic used for soldering is small stability of the joint under temperature cycling conditions, this being connected with easy formation of cracks in soldered joints when their thickness exceeds 0.2 mm.
A high-temperature composite material is known [U.S. Pat. No. 4,970,179, NPC 501-92], consisting of a silicide matrix and silicon carbide dispersed therein. Molybdenum disilicide occupies 50-90 mole percent of the matrix and the remaining portion thereof is occupied by at least one refractory silicide selected from the group consisting of WSi
2
, NbSi
2
, TaSi
2
, Mo
5
Si
2
, W
5
Si
3
, Nb
5
Si
3
, Ta
5
Si
3
, Ti
5
Si
3
, TiSi
2
, CrSi
2
, ZrSi
2
, YSi
2
. Silicon carbide occupies 10-30 volume percent and is in the form of submicron powders or whiskers (elongated single crystals) or a mixture of these forms consisting, mainly, of particles with a diameter of 0.1-2.0 &mgr;m. As pointed out in the specification, an insignificant amount of (Mo,W)Si
2
solid solution may be present in the material.
The main disadvantages of this composite material are: low resistance to crack formation and subsequent degradation under temperature cycling conditions with rates higher than 20° C./second in connection with high the content of molybdenum disilicide in the material. Attempts to use this material as a solder will inevitably lead to degradation of the submicron particles of silicon carbide present in the material.
The prior art most relevant to the proposed invention (prototype) is the known composite high-temperature and heat-resistant material “REFSIC” [RU 2160790 C2, C22C 29/18, H05B 3/14, C04B 35/58] comprising silicon carbide and disilicides of molybdenum and tungsten in the form of MoSi
2
, WSi
2
, (Mo,W)Si
2
, Mo
5
Si
3
, W
5
Si
3
, (Mo,W)Si
3
and/or Mo
5
Si
3
C and/or (Mo,W)
5
Si
3
C phases with the following ratio of the components (vol. %): Mo
5
Si
3
and W
5
Si
3
and/or (Mo,W)
5
Si
3
and/or (Mo,W)
5
Si
3
C and/or Mo
5
Si
3
C, 15-85; tungsten and/or molybdenum disilicides WSi
2
and MoSi
2
and/or (Mo,W)Si
2
, up to 55; silicon carbide, 2-85; the content of molybdenum and tungsten in the total mass of the refractory metals in the silicide phases of the material is in the ration (in wt. %): Mo, 7-80; W, 20-93.
The main disadvantages of the prototype material are connected with difficulties in using thereof for providing soldered joints and protective coatings due to the presence in it of skeleton (coherent) structures composed of grains of silicon carbide, whose volume fraction may reach 85%. It is just the cohesion of silicon carbide grains in the “REFSIC” material that provides its heat resistance up to temperatures of 2000° C. and higher. However, it is just the cohesion of the silicon carbide skeleton that rules out the possibility of complete melting of the “REFSIC” material at temperatures below 2000° C., most often required in soldering. It is practically inexpedient to remelt “
Gnesin Boris Abramovich
Gurzhiyants Pavel Artemovich
Institut Fiziki Tverdogo Tela Rossiiskoi Akademii Nauk
Ladas & Parry
Patel Vinod D.
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