STUDY OF THE MICROSTRUCTURE OF CERAMICS
FROM DIFFERENT PRODUCERS USED FOR PROTECTING
PRECIOUS METAL THERMOCOUPLES
I. L. Shkarupa,
L. A. Plyasunkova,
and A. A. Ulanovskii
Translated from Novye Ogneupory, No. 7, pp. 31 – 35, July 2008.
Original article submitted April 25, 2008.
Results are provided for a study of the microstructure of ceramics from four different producers by means of a
scanning electron microscope and x-ray microanalysis. Results show that ceramic specimens often do not
correspond to claimed parameters. The main disadvantages are ceramic structural inhomogeneity, presence of
phases of a different nature, intergranular pores and cracks, and alkali impurities. The ceramics of only one
producer correspond to the main parameters for this class, and this is provided by high production technology
and starting material quality.
Thermometry by means of thermocouples based on
precious metals under conditions of a continuous increase in
the price of platinum and rhodium requires reliable protec-
tion of thermocouple electrodes in corrosive high-tempe-
rature atmospheres. A long working life for the thermal
converter is a condition for its competitiveness.
The most important characteristics of ceramics for
protecting thermocouples are structural uniformity, grain
size, absence of pores, cracks (gas tightness) and binder
residues, and thermal stability. The composition, thermo
physical and electrotechnical properties of ceramics are
standardized by international standards IEC 672-1–80, IEC
672-3–84, and in Russia by interstate standard GOST 20419.
In order to protect thermocouples there is use of ceramic
types S530, S610, S795 and S799 based on Al
, and also
ceramics produced under various trade names, but corres
ponding to the same types, for example especially dense
ceramics Rapal 100 or Alsint 99.7 (S799), Pythagoras
(S610), Sillimantin (S530). Comparative characteristics of
ceramics of different types are provided in Table 1 (accord
ing to producer data).
It can be seen from Table 1 that porous ceramic with
stands the maximum thermal shock, but it does not have a
capacity to protect the internal volume of a sheath from
chemical action of the environment. Of the gas-tight ceramics
the most resistant to thermal shock is mullite-corundum
ceramic S610, consisting of a mixture of Al
With an increase in Al
content in the ceramic there is an
improvement in its strength properties, thermal conductivity,
and an increase in the maximum working temperature and
chemical resistance, but a reduction in thermal shock resis-
tance. Therefore a standard sheath for a thermal converter
with a thermocouple made of precious metals is made double
(Fig. 1). The outer sheath is made of gas-tight or porous
ceramic with resistance to thermal shock and inner sheath is
made of gas-tight ceramic with a high Al
content. With a
working temperature above 1500°C there is use of ceramics
with the maximum Al
content and density. These
ceramics require particularly careful treatment during
operation. An important rule is to select the type of ceramic
in relation to its application conditions; the most expensive
ceramic may not always provide the required operating life.
The aim of this study is to analyze the structure, phase
and elemental composition of ceramics from different
producers in order to determine their suitability for protect
ing platinum-rhodium thermocouples. Microstructural analysis
was performed by means of a scanning electron microscope
EVO-40 and a QUANTAX x-ray microanalysis system. For
this study the following specimens of ceramic objects were
producer 1for ceramic type S799, sheath diameter 12.5
and height 8.3 mm;
Refractories and Industrial Ceramics Vol. 49, No. 4, 2008
1083-4877/08/4904-0272 © 2008 Springer Science+Business Media, Inc.
FGUP ONPP Tekhnologiya, OOO OTK, Russia.