SYNTHESIS OF ELECTROCONDUCTIVE CERMET MATERIAL BASED ON Zn2SnO4

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SYNTHESIS OF ELECTROCONDUCTIVE CERMET MATERIAL
BASED ON Zn2SnO4
Institute of Structural Macrokinetics and
Materials Science, RAS Chernogolovka, Moscow,
Russia ?.V. Barinova, I.P. Borovinskaya, V.I.
Ratnikov, A.F. Belikova

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Some requirements to the material of an inert
anode for aluminium industry.

• Wear rate lt 1
  cm/year
• Specific resistance lt 50 ?Ohm m
• Specific resistance change lt 10 /year.
• Thermal stability - 100
  thermal cycles
• Compression strength 50 MPa
• Fracture strength 1.5 MPa
  m1/2
• Conservation of dimensions at 960 1000?
• Machinability
• In order to obtain the anode material with the
  mentioned characteristics we chose the system of
  Zn-Sn-Ni-O. The oxides of this system are
  characterized by high chemical inertness to
  various electrolytes and oxygen. In this system a
  complex oxide with the spinel structure, i.e.
  zinc orthostanat Zn2SnO4 can be formed. This
  compound is a dielectric, but it is characterized
  by high corrosion resistance to aggressive media
  at high temperatures. Some metal additives (for
  example copper) were supposed to be introduced
  into the oxide ceramics, in order to get the
  preset specific resistance, 50 ?Ohm m.
• The aim of our work was to obtain cermet
  materials in the system
• Zn-Sn-Ni-O which should be characterized by
  high thermal stability
• and corrosion resistance and the preset
  specific resistance,
• i.e. 50 ?Ohm m.

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• The synthesis conditions
• The scheme of the reaction
• 5 Zn 3 NiO 2SnO2 ? 3 ZnO Zn2SnO4
  Ni3Sn
• The powder components of the green mixture
• Zn , Sn, ?u, ZnO, SnO2, NiO, Cu2O, CuO
• The particle size NiO up to 50 ?m and up to
  175 ?m.
• The material was obtained by the method of SHS
  compaction
• - the combustion temperature ?c 1150º ?
• - the compaction
  pressure 30 ?P?
• - The density of the billets
  ranged from 4.2 to 4.9 g/cm3
• The final product was a dense cylinder of
  30 mm in diameter

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The ?ermet material obtained by using nickel
oxide powder with the particles size of up to 50
?m, ?? -1150º?
The product contains Zn2SnO4 , Zn?, NiO,
Ni3Sn, Ni-Sn The cylinder of product had a
fracture circular structure. The outer case of 4
mm in width consists mainly of ZnO-based phase.
The second zone is based on ZnO and NiO.
The central zone consists of spinel Zn2SnO4
with metal inclusions as well as phase inclusions
based on ZnO and NiO. The porosity in the center
of the sample is from 7 up to 10 . The
specific resistance is 1100 ?Ohm m in the
central part formed by Zn2SnO4 and 8600 ?Ohm m
in the outer area formed by ZnO.
Fig.2. Structure of the sample (?2000)
Fig.1. Photo of the cross section.
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Table 1. The values of specific resistance
(?, ?Ohm?m)

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The ?ermet material obtained by using
nickel oxide powder with the particles up to 175
?m in size , ?c 1150º? The product contains
Zn2SnO4 , Zn?, NiO, Ni3Sn, Ni-Sn In this case a
textured material was obtained. The texture was
determined by elongated dark particles with a
light shell which were oriented perpendicularly
the pressing axis. The specific resistance of the
obtained product measured in the central part in
the direction perpendicular to the compaction
axis was 80 86 ?Ohm?m. Introduction of 8
copper oxide into the initial mixture and
realization of the synthesis under the terms
providing texture formation allowed decreasing
the specific resistance to 26 ?Ohm?m.
Fig.1. Structure of the sample Zn-Sn-Ni-O, with
oriented particle ( texture)



Fig.2. The oriented particle
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The ?ermet material used nickel oxide powder
with the particles size of up to 175 ?m , ??
1300º? The product contains Zn2SnO4 , Zn?,
NiO, Ni3Sn, Ni-Sn. Under such terms, the
material phase composition and texture remained
the same, but the texture was determined not by
the shelled particles, but by the elongated
formations, consisting of a metal phase Ni-Sn,
dispersed in ZnO-based phase. The specific
resistance of the obtained product is 1050 ?Ohm?m.
Fig. 1.The elongated formations consisting of a
metal phase Ni-Sn dispersed in ZnO-based phase.
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The initial test results of the obtained
materials. Fig. a,b,c demonstrates the sample
Zn-Sn-Ni-Cu-O before and after thermal and
corrosion resistance analysis.
a. The initial sample
b. The sample after thermal stability test
(960ºC, ? 5 hours)
c. The sample after polarization and
electrolysis (960ºC, I 500 mA/cm2, Al2O3
Na3AlF6, ? 1 hour)
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Conclusions
1.Cermet material based on Zn2SnO4 was
synthesized by the combustion mode. Dense
cylindrical samples were obtained they consisted
of ZnO-based dense shell and an electroconducting
layer in which the metal phase was distributed in
the chemically and thermally stable oxide phase
based on Zn2SnO4. 2.The electro-conducting
properties of the cermet material can be achieved
during the synthesis due to the development of
such structural peculiarities as elongated
particles oriented perpendicularly the pressing
axis and possessing metal shells. 3.The metal
shells were shown to consist of Ni-Sn solid
solution. 4. The main metal phase responsible
for obtaining the material with low specific
resistance is Ni-Sn solid solution. A decrease in
the specific resistance was observed when Ni-Sn
metal phase consisted of long filaments oriented
perpendicularly the pressing axis.
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Table 2. Some requirements to the material of an
inert anodeand the results of the tests