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Tin-Oxide, bulk-ceramic resistors are...
Tin-Oxide,
bulk-ceramic resistors are made from tin oxide, antimony and glass. Tin
Oxides (SnO2) have been used in resistor
manufacturing
for years. Most thin-film resistors are made by depositing a thin layer of
tin oxide onto a ceramic core and
spiral
trimming to desired value. In pure SnO2 conductivity is directly
proportional to free electron concentration
(s
= [e-] q µ); thus the conductivity will depend on partial oxygen pressure
and is expected to sharply rise with temperature
(making
TCR unacceptable for resistor application). The addition of antimony is a
proven way of limiting the TCR effect of
pure
tin oxide. The conductivity model of SnO2 doped with Sb2O3 (Equation 1)
shows free electron concentration independent of partial oxygen pressure
and
temperature.
Research has shown that maximum conductivity is reached at 5-10% antimony
in tin-oxide. Our experiment proved this study to be correct and found
conductivity to
peak
at 5% (Figure 1).
Moreover, as expected, TCR decreased with increasing antimony content
(-1.90E-2 pure SnO2 to -7.20E-5 for 5% Sb2O3 content).
The tin oxide/antimony composition showed good "resistor
properties"; however, by itself it has a very limited resistance range
and is fairly brittle. A glass phase is
commonly
used to bond tin oxides. We chose two commercial frits and a designed
series of experiments to illustrate the resistance and TCR range of
tinoxide/glass
matrix.
Samples were prepared by grinding and mixing frits with tin-oxide/antimony
at given volume-to-volume ratios, pressed to 0.5 in. pellets and cintered.
Networking
of tin-oxide particles controls conductivity in the matrix. It can be seen
in Figure 2 that light phase, tin-oxide particles are better networked
after 1200°C
firing
than 700°C; hence conductivity increases with cintering temperature.
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