A HEATING MODULE EQUIPPED
WITH LANTHANUM CHROMITE-BASED HEATERS
S. A. Suvorov
and A. P. Shevchik
Translated from Novye Ogneupory, No. 3, pp. 23 – 28, March, 2004.
A heating module for service in oxidizing gas media at temperatures up to 1700°C is proposed. Results of an
analysis of the thermally stressed state of lanthanum chromite-based heaters designed in various configura
tions for use in the heating module are reported.
High-temperature technology based on the use of oxidiz
ing gas media has found application in the manufacture of a
wide range of materials and products with unique optical,
electrical, magnetic, and other properties, both in research
and industry. Electric resistance furnaces used for that pur-
posed are equipped with high-temperature heaters [1 – 4].
Metals and metal alloys as heaters in oxidizing gas media
have a limited application — as a rule, at temperatures not
higher than 1300°C. Most commonly, oxide-based (lantha-
num chromite, zirconium dioxide) or oxygen-free materials
(molybdenum disilicide, silicon carbide) are used for heating
in oxidizing media. The service life and maximum tempera-
ture for oxygen-free nonmetallic materials are controlled by
irreversible oxidation processes. Oxide materials have an
electric conductivity, as a rule, that is insufficient for resis
tive self-sustained heating from room temperature, which
constraints their use as heaters. For example, heating ele
ments from ZrO
must necessarily be preheated to 1000 –
1200°C before they are able to conduct electric current.
Materials based on lanthanum chromite combine unique
properties rarely found in high-temperature oxide materials.
They have a high melting point (about 2500°C), chemical re
sistance to gas media with different partial oxygen pressures,
and high conductivity in oxidizing gaz media. Materials
combining such properties have found use in the manufac
ture of solid-electrolyte fuel cells, heaters for electric fur
naces, electrodes for magnetohydrodynamic generators, etc.
[5 – 9].
Lanthanum chromite heaters, evaluated in terms of cost,
operating temperature, service life, and surface thermal out
put, are quite competitive with heaters based on silicon car
bide and molybdenum disilicide. The main parameters to
characterize electrical heaters are:
– total resistance at 20°C;
– specific resistance and temperature coefficient of re
– maximum operating temperature — a temperature at
which the heater’s service life is about 1000 h;
– permissible load — power output per unit surface at
which the heater sustains no damage;
– maximum permissible heating rate;
– voltage-current characteristic — relationship between
the voltage drop across a heater and the current passing
– working gas medium.
The composition and properties of high-temperature ma-
terials for heaters inevitably alter during service. A specific
feature here is that the evolutionary processes take place in
cycling temperature regimes and under voltage applied. The
temperature stresses that occur under such conditions have
an effect both on composition and properties of the material.
Thermally stressed areas in the resistive material serve, as a
rule, as sources for the occurrence of microcracks, which re
sults in local disturbances of electrical conductivity. Finally,
the heating element becomes prematurely disabled. A means
of identifying areas prone to failure may be a numerical anal
ysis of the temperature field and the thermal stresses associ
ated therewith in the lanthanum chromite material under re
sistive heating conditions.
Mathematical models for the thermomechanical behavior
of continuous media are constructed by means of partial dif
ferential equations. The differential equations with the ap
propriate boundary conditions form boundary-value prob
lems of mathematical physics. Two and three-dimensional
boundary-value problems, as a rule, can be solved only ap
proximately using various methods. Among these, the finite
difference method and variational methods (Galerkin, Ritz,
finite element methods and others) can be mentioned.
Refractories and Industrial Ceramics Vol. 45, No. 3, 2004
1083-4877/04/4503-0196 © 2004 Plenum Publishing Corporation
St. Petersburg State Technological Institute (Technical Univer
sity), St. Petersburg, Russia.