Compositions – Electrically conductive or emissive compositions – Metal compound containing
Reexamination Certificate
2000-02-28
2001-03-13
Kopec, Mark (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Metal compound containing
C252S520200, C252S520500
Reexamination Certificate
active
06200501
ABSTRACT:
The present invention relates to the field of producing high-temperature ceramics and can be used in manufacturing high-temperature heaters, semiconductors, thermocouples, temperature sensors, as additives in manufacturing conventional ceramics and in other fields requiring high thermal and chemical stability and high electric conductivity when operating in air, and in medical practice.
An electroconductive material is known on the basis of rare-earth chromites, such as lanthanum and/or yttrium chromite with additions of zirconium dioxide and rare-earth oxides, which is intended for stable operation at high temperatures, about 2000° K. In this case the amount of zirconium dioxide must exceed 5 mol %, preferably 30-50 mol % (see U.S. Pat. No. 3,475,352, “Electrically conductive ceramic material”, national classification 252-520, published Oct. 28, 1969, which is taken as the analogue).
Better results are obtained with compositions containing 33 mol % LaCrO
3
, 16 mol % Gd
2
O
3
, the rest being ZrO
2
(Composition 1) and 33 mol % LaCrO
3
and the rest, ZrO
2
(Composition 2). Hereinafter, the properties of the novel material are compared with those of the ceramics of the two compositions.
A serious shortcoming of the said ceramic material is its low permissible rate of heating, relatively low reliability of operation at 1600° C., and low thermal stability when operating at about 1500° C.
The closest analogue of the proposed invention which is taken as the prototype is an electroconductive ceramic material disclosed in patent application PCT/US93/05818, published WO 93/26011, Dec. 23, 1993, which contains the following ingredients, wt %:
MgAl
2
O
4
0.5-10.0
MgCrO
4
1.0-15.0
CaZrO
3
up to 10.0
YCrO
3
up to 5.0
ZrO
2
up to 5.0
CeO
2
up to 1.0
LaCrO
3
the rest
A shortcoming of the prototype is its relatively low emissivity.
The object of the present invention is to make an electroconductive material capable of being heated at a fast rate, having high reliability in operation at high temperature, and having high emissivity.
The goal is achieved by supplementing an electroconductive ceramic material containing magnesium chromite MgCrO
4
, yttrium cliromite YCrO
3
, zirconium dioxide ZrO
2
, cerium dioxide CeO
2
, lanthanum chromite LaCrO
3
, by lanthanum aluminate LaAl
2
O
4
, in the following ratio of ingredients, wt %:
lanthanum aluminate
LaAl
2
O
4
0.5-10.0
magnesium chromite
MgCrO
4
1.0-15.0
yttrium chromite
YCrO
3
0.5-3.0
zirconium dioxide
ZrO
2
0.5-5.0
cerium dioxide
CeO
2
0.1-1.0
lanthanum chromite
LaCrO
3
the rest
The material was obtained in the following manner. After preparing a charge of the necessary composition it was mixed in a planetary mill using Plexiglass drums and Teflon balls as milling bodies. The resulting powder was dried and melted in a solar furnace. The melt was cooled, ground and pressed into specimens of 50×6×6 mm in the middle and 50×6×12 mm at the ends for measuring maximum heating rate, of 40×4×4 mm for measuring electric conductivity, and of 15 mm in diameter and height for the rest of the tests. The specimens were sintered in a lanthanum chromite furnace it 1600° C. for 12 hours. The specimens thus prepared were held at 1500° C. for 60 hours for determining thermal stability at that temperature, and at 1600° C. for 20 hours for determining weight loss. To measure conductivity, the specimens of 40×4×4 mm were metallized at the ends.
To measure maximum heating rate the appropriate specimens were put in a corundum chamber and, after having a thermocouple attached to them, were energized and heated at various rates. Then the specimens were examined on a cross-section. When a certain heating rate was exceeded the specimens disintegrated owing to cracking at the surface and the inner layer melting through because of the low thermal conductivity and inverse temperature dependence of electric conductivity. The results obtained for various ceramic compositions are listed in Tables 1 to 11.
REFERENCES:
patent: 3475352 (1969-10-01), Barbier et al.
patent: 5864576 (1999-01-01), Nakatani et al.
patent: 93/26011 (1993-12-01), None
Abelman ,Frayne & Schwab
Kopec Mark
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